Personally I'd go for an ARS rack system, the rack that the pros use. You'd probably put a bit more into an ARS rack system but when you sell it you'll get way more than a home made rack system. Plus you'll have access to different kinds of tubs - grey, clear, transparent, no cup holder, with cup holder, etc. You can go with an ARS-7010 and just buy the levels you want. They run $100 per level, big enough for the biggest ball pythons. I like the versatility of the rack system too, I have a modified ARS rack system with several rows of ARS-7030 and several rows of ARS-8018, they actually stack right on top of each other. And I have the wheel base too so I can easily move it around the room. The ARS-7030 system is actually cheaper per tub, it's $200 per level ($200 per level with three tubs instead of $100 with one tub). If I were you I'd get two levels of ARS-7030 with a wheel base. You can always add on later.
With an ARS rack system you'll never have to worry about proper ventilation or snakes sliding out the tubs. I keep even the smallest snakes in ARS-7030 tubs with no problems at all and they can grow up into adults in the same tub, I just can't keep as many small snakes since the big tubs take up so much room. I don't have dividers or hides and my baby ball pythons do great and eat frozen thawed twice a week.
Here's a few photos, ARS-7010 on top, ARS-7030, and my snake room and rack in the back that holds 20 snakes.
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