Thanks for the link there. I may end up doing one of those. I'm also looking into prices to see if I can't order a sheet of expanded PVC or some other sheet of plastic. If I could do that, I could glue/silicone/(pvc weld?) it right in to the plastic frame of the aquarium, and put hinged or sliding doors on it.
And for ventilation, yeah, there won't be a ton of ventilation. But I can make it with just as much ventilation as you'd find in any of the plastic cages. Boaphiles specifically come to mind- my boaphiles only have a small gap around the door for ventilation, and they work fine. If I ended up really needing to, I could always drill the back/bottom of the aquarium and make that into a vent. If I ever ended up using the aquarium to hold water again (doubtful) I could just use that kind of hole as an overflow.
I want to lay it down because if I can lay these two tanks down, I can build a stand/rack to hold them and use them. If I can't, I'm going to have to sell them (and old 55s that don't hold water are worth . . . $20 each?) and buy cages that will work. And I really don't have the money for that right now. I've been trying to put all of my money into making things look nicer in the snake room (building racks & stands, decorating cages, etc. . .) and I really don't want to have to spend more money on another cage that's going to sit on the floor.