Another thought for utilizing space in that tank for a boa cheaply, is some type of shelf. He probably won't "perch" much, but most boas enjoy sitting up on top of things sometimes. Also gives him more temperature ranges relative to the basking lamp. I've found cheap "over the sink" and "over the counter" shelves to work well. This is one is in my bredls enclosure. Dollar store, $4 I think. Its made of some type of unknown wood, so it has a couple of coats of helmsman spar urethane on it, which also makes it much easier to clean.
You'll have to look around and see what you can find. You can also construct the hide underneath if you'd like. I've got some greenery there cinnamon likes to hide behind some days. But she's in the living room, and rather enjoys sitting up there watching the world. She's obviously not a boa, but allot of boas have a very similar personality.