Hi Brett,
If light weight is your primary concern then building your own is probably not the best idea. You could build the racks from 1/4" expanded PVC but I don't think that is the best material for small colubrids or live-bearing snakes such as your Rosy Boas.
I can think of two racks that are light and would be fairly secure for rosy boas and small colubrids. My favorite is that available from Jason's Jungle. He keeps rosy and sand boas and his racks, constructed from 1/2" expanded PVC, are excellent for them.
Another rack is Doug Barr's - herpcages. com - they should be very light and secure. I have not seen these racks in person, however, so I can't make a full recommendation for the particular species you keep. I suspect they would work fine.
Be forwarned that many of the other plastic racks on the market are very heavy and/or not secure enough for what you keep. Some plastic sheets weigh as much or more than plywood so don't fall into the trap of assuming that any plastic rack will be light.
At any rate, below is a link for building a hatchling rack. I like the link because it shows building a rack upside down - the way I like to do it.
http://arbreptiles.com/cages/rack.shtml
But I don't like to use the boxes as spacers. Instead I cut the spacers from scrap melamine. Scroll down to step 2 in the link below to see the melamine spacers. I use these instead of the boxes - again building the rack upside down.
http://finegtps.com/Racks.htm
For your racks I would find the rubbermaid 28 qt. boxes or the Stertile 32 qt. boxes. Sterilites 28 qt. box would also work but I don't like the raised feet on the bottom. This will make the boxes less secure for the species you keep.
The rubbermaid 28 qt. box will allow you to fit 7 boxes in a 4' tall rack. The sterilite 32 qt. will allow 6.
Have the large boxes slide in width-wise. This will allow you to use two half-sized boxes on any level.
You should be able to see that in the picture below. Some levels have one large box, others have two half sized boxes. This also makes for a slightly lighter rack.
This particular rack (which was only holding reptile supplies when the picture was taken) has 1/2" plywood sides and 3/4" melamine shelves. For a back I taped on a piece of foil-covered, strene insulation. Heat tape can be attached here as well for heat. It cost me about $40.
