This looks like the best bet with EasyAVR then?
Two thoughts: One, I have one like that and it has fewer pins than the teensy -- you'll want to count your projects rows & pins to make sure you have enough without getting really creative to invent -- what, a 6x6 matrix? The ebay Desc. says 12 DIOs, but that's strictly Digital I/O ... there's a few Analog I/Os too. But that's still an 8x8 matrix, which equates to 12x4 the easy way. Not quite a 60% board.
The other thing is, using it with this software, the pins will be at slightly different places (I think) so you're going to need to use the 'handwire' option ... or TMK which is much more flexible in that regard.
Otherwise, yeah. It'll work fine.