Well, one thing a friend of mine had he made himself. I havn't made one for me yet but I might...
Take a small submersible pump, like used in a small pond or large fountain. Take a small containter that will give you easy access and fill it with media of choosing (he has used plain cotton for his in-between filtering). use maybe a small tupperware bottle (like used for ketchup etc). You modifiy the lid to accept your vacuum hose (gravel vacuum not shopvac). In the bottom of the containter, modifiy it to mount onto the inlet of the submersible pump. Then take another length of hose on the pumps outlet for the clean return.
So there's no accidental spill, he takes a shop clamp and clamps on the return hose onto the side of the tank first. Then drops his gravel vac into the tank, and away he goes. The water is filtered before it hits the pump so there's no gunk or buildup in its impeller. Cleaned water goes back in tank. When he's done, he unhooks it all and tosses out the media. He says cotton wads work pretty good.
OR, you could also go on Craig's list/eBay etc and find a small canister filter and basically do the same thing. Something you use for your in-between cleaning.