I haven't come across any 'free' do it yourself cage building plans/sites. There have been some articles on building cages in various Reptile Magazines. Might be able to find the articles on their website. If not the articles themselves, then at least, which issues they appeared in.
Searching the Enclosure forum and searching various forums will bring up people's posts on building cages. Though you are bound to find cages for all kinds of animals, the basic principals and steps will be the same. Just remember the minimum size you need for bearded dragons is 4'x2' of floor space with height between 18"-24" if you plan to use florescent tube style UVB bulbs. If you are going for mercury vapor bulbs, then make the height no less than 24" (could go as high as 30" but to me, that would be more wasted space....)
The way I have designed my cages are pretty simple. As I suck as a carpenter and don't have access to any 'cool' tools. (ie a router, skill saw/table saw, etc.) I only have a power drill and a hand saw.
Thus...I drew out the basic layout of my cage on graph paper (graph paper isn't needed, but it certainly makes it easier to ensure everything is square, and the grid can be used to represent feet, or each square can be quarter foot..etc) Or you can just use a ruler and measure out dimensions such as one inch = one foot...and go from there.
Basically, you are making a 5 sided rectangular box. The sixth side will be the door. I design my cages to be 5'x2'x2' in size (I like giving my dragons extra space). Thus, you need the following sizes of wood cut:
5'x2 - 3 of these (top, bottom, back of cage)
2'x2' special - 2 of these (left and right side)
Note to make though. Because wood has a thickness...the end 'caps' will actually be larger than 2 feet square.
THus, if you are using 1/4" plywood (or melamine/particle board/pressed board/hard board...stuff has more names than anything, basically sawdust with glue, covered in a vinyl coating), the two end pieces will need to be 1/4" longer on all dimensions (thus 2' 1/2" x 2' 1/2" you have to add the quarter inch on both left and right sides, and top and bottom sides of the end caps, which gives you the half inch (two quarters of an inch added together, equals a half inch)
As I have no power tools to cut sheets of wood (not to mention not owning a truck) I had the hardware store do my cuts for me. IT takes them about 5 minutes to do it...much better to get them to do all the work.
I pre drill all my holes. Much easier to screw the cage together.
Just make sure your drill bit isn't wider than the width of the screw shaft..else it won't hold. It is better for the drill bit to be a bit smaller...the screw will bit in better.
For the door, I just built a frame out of 1"x4" boards, stapled an aluminum screen over it with spare trimming.
The screen on the doors will allow heat to escape but not as much as on the top. Vents can instead be cut on the sides the glass sliding doors can be used instead. But I don't particularly like glass doors, too easy for stuff to get into the tracks, etc. But, because my apartment rarely drops below 73F at night, even in the winter...the screen door shouldn't present a problem with temps, even during the night.
For light fixtures, I bought a standard shop double bulb fixture for UVB lights, 4 feet long (thus, it covers nearly the total length of the cage...and still fits in). I bought the one that already has a plug, as opposed to the kind you wire directly to your household power. I also bought a ceramic flat fixture for the heating bulb (basically the plain kind you find often in basements..which don't have any pretty glass 'bowls' to cover the naked bulb). I drill holes for the basking light fixture, so I can mount it directly inside the cage, and have the wires outside through the hole (I just hand wire to a double cord, with a plug at the other end. Use electrical tape to cover screws and secure the wires, or attach a 'cage' around the back of the fixture (those ugly steel boxes you see all over the electrical section of a home hardware store). Follow instructions in the box that comes with the fixture.
The florescent fixture I suspend from the inside using the Eye hooks it comes with. Hook the chain so the fixture is low enough over the basking area to expose your dragon to UVB light, but height enough he can't jump up at it. Typically for a two foot high tank, 6-8" from the top of the fixture to the underside of the top of the tank, is good. Height of the basking spot itself, will determine exactly how low the fixture needs to be.
For florescent bulbs, I use one Reptiglo/repti-sun UVB 8.0 (or 10.0 if you have them available) and one full spectrum plant tube (you can find these at the hardware store, the UVB light will have to be ordered online or picked up at your nearest pet store or reptile show). The full spectrum is mostly to give additional UVA (it may put out really low levels of UVB but not enough to be worth using this light exclusively for the health of your dragon) and just plain light, to keep the rest of the cage fully light during the day. A hole can be drilled through the top of the cage, or side, for the plug of the florescent light to be pulled through.
Both lights I plug into a power strip, then plug the power strip into a heavy duty indoor timer (usually because power strips using 3-prong plugs and only the heavy duty indoor timers have three prong outlets in them. If you don't want to go with the power strip, you can use a typical extension cord with 2-3 outlets, and plug that into a normal indoor timer).
As I use plastic table cloth for my substrate, I don't worry about sand getting out of the cage, so i didn't worry about creating a 'sand barrier' at the bottom of the cage. If you plan to use sand, then build a 2-6" 'foot' along the bottom of the cage, then set your door to fit in the remaining space.
Important tips. Measure twice, cut once.
Don't forget the thickness of the wood for the end pieces.
Seal the cracks inside the cage, once finished, to prevent stuff getting into the cracks and thus, becoming a haven for bacteria (or even mites, if they should get into your collection) to grow.
Build a mock up in basel wood or scrap wood,if you are not sure how to get everything together. Scale models are fun to build 
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PHLdyPayne
