Does your room have a concrete floor?
Have you considered Sponge filters? (lots of small pumps or hoses and valves)
Have you considered a central canister filter? (lotta hoses and valves)
Cheapest sponge filter you can do is a section of 1 inch bubble lift pipe about six inches long, a larger (pondmaster's smallest) intake pre-filter sponge and a nice tom's top deflector. This results in an eight inch tall two inch diameter sponge filter.
You could pop $10.00 each and get a few sponge filters (shipping figured in price for about four). You can run four of them with a valve manifold off a standard air pump.
The canister filter solution is a little trickier and requires some hosing, valves and clips to hold the hoses in place and also sets up the situation where multiple fish are exposed to pathogen at the same time. A C-160 would likely run ten two gallon tanks without a problem (some flow loss from turbulence in the hoses)
In multiple divider tanks you can use a hose stuck into the intake run over to the other end to complete a circulation cycle through the dividers. (pre filter sponge over the intake tip on that end)
If you want to spend "too much" you could go with several whisper 3i... etc.
It depends on how you wish to maintain the tanks in the end.
switch to "round" These take very little air to operate and can be trimmed down to about half their diameter AND burried in the rocks. They'll support cycling of the tank so you can get away with "remove betta, drain, fill, replace betta" tank cleaning instead of full scrub outs. IIRC you can make Lee's small undergravel carbon heads fit on top too if you want refillable media baskets for em
Can't find a sales link but here's a horrible little contraption with a full undergravel filter that has a re-fillable carbon and floss head. http://www.bigalsonline.com/Fish_Aqu...tml?tc=default
(the oval version isn't quite so horrible)