Personally I think the entire cage should not be all plexiglass. It will end up being more expensive than just using wood or regular glass. (only difference would be weight).
Plexiglass does scratch up and will warp if it gets too hot..depending on the thickness. Also, some dragons can really become stressed with too many clear sides. I had a cage built with plexiglass on the ends and part of the front and screen doors on front. The rest was wood though half the top was wire mesh, the other half wood. I attached the UVB fixture to the underside of the roof part and the heat lamp inserted through an opening of the wire mesh. The new cage I built has both light fixtures on the inside of the cage...the UVB fixture suspended on chains so its lower..and better able to provide UVB (as I use the florescent tube style).
The old cage plexiglass sides did get scratched up pretty quickly but the wooden frame help protected it some I believe. Below are some pictures of the old cage.


Oh to cut down on weight and cost, the back and bottom are actually floor inlay sheets instead of the more heavier plywood. Never had problems with sagging but if you plan to put sand in, then go with the plywood or at least run some lengths of either 2"x2" or 2"x4" boards along the bottom for additional support.
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PHLdyPayne