It's been my experience when building cages, you should build the cage to the size that the snake will need as an adult. snakes grow like weeds and can quickly out grow living quarters. Unless you like building things, have a lot fo free time, and money to burn. Build away.
http://wwhardware.com/catalog.cfm/GroupID/Sliding Door Hardware/CatID/Epco Sliding Glass Door/showprod/1
that's what I use for my sliding glass doors. Kits are reasonably priced and easy to work with. Cut to length, order your glass, and you're all gravy...
Now screened tops on a home made cage can be made, however, I would recommend using the floor radiator vents on the back or sides of the cage. there's a couple builders on here that use them. Open screen tops let a lot of heat out of your enclosure, causing your equipment to work harder than it has to. With a couple good vent holes in the sides, a good proportional thermastat, and a heat lamp/UTH... you should be golden...
Hope I helped...