ack. sorry, didn't mean to discourage you.
My first paludarium was in a 10-gallon, I siliconed lava rock allover the back, and built a ledge and a waterfall out of it. pretty low-skill, but took a bit of time, and a lot of silicone, and the tank was quite heavy when done.
you mentioned you weren't going to put any fauna in it, how about some sort of riparium? that would give you plenty of water volume, you could still use a HOB, and you could either use rafts, hanging planters, or even build a small ledge up high.
You already need to get lights, so maybe you could do a CFL brooder suspended over the tank or something.