I found an interesting design for a very simple air pump online. Looks only to be suitable for indoor aquaria, but still intriguing. The design is the large image at the middle of this page: http://rcdening.co.uk/nat_history_notes/aquaria.htm

The "reversed Bunsen valve" is just a one way valve letting air into the bottle from outside the aquarium. A bunsen valve design can be found at the bottom of the first page here: http://www.rsc.org/images/cracking_tcm18-188813.pdf

I'm looking into a way to make one of these affordably here. Points I'm thinking on at the moment are bottle size (old jam jars likely, though canning jars would be good too), cork options (want to avoid metal jam jar lids) and how to drill a hole in the cork if we go the rubber cork route recommended, as well as air outlet pipe size (smaller bubbles = better diffusion into the water, but harder to make and likely slower bubble rate)