There are numerous rack-building links on the web. I'll try to name a few at the end of this post. Understand that many keepers use different sized boxes due to local availability, boxes being discontinued, etc. So it's likely you'll have to modify any plans for the specific box you find and prefer.
With that said..
I build my racks upside down like you see on the hatchling rack plans at ArbReptiles.com.
But I use a cut piece of melamine for a spacer rather than folded paper. You can see that at FineGTPs.com.
More rack plans/ideas can be seen at BallPython.ca and RioBravoReptiles.com.
Honestly, racks are not that difficult to build provided the material is cut straight, square, etc. In fact getting the material cut at a cabinet shop (vs. Home Depot or some place like that) is often a good idea. On the other hand, you really don't need anything more than a circular saw and an edge-guide to achieve acceptable results. Well that and a drill for assembly.
My main piece of advice is to not make the typical beginner mistake of making the gap between the top of the box and the underside of the above shelf too small. I say this for all snakes but especially for BP's which are not the best escape artists.
It's always better to make the gap too big as it's easy to add material than it is to remove material in this case. Trust me on this.
You'll want 1/8" to 3/16" for a BP. I'd go with 3/16" for subadults, 1/8" for hatchlings.
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Current snakes:
1.1 Gonyosoma oxycephala - (Silver/Yellow)
2.2 Gonyosoma oxycephala - (Green)
4.3 Gonyosoma janseni - (Black)