There's really no such thing as guitar or bass effects (at least in my eyes). Sometimes, you can lose the low end for the sake of the right effect. That being said-- I've seen plenty of mods, but not a whole lot of bass-oriented effects for DIY guys.
Distortion is really the only thing you have to worry about, though. Most modulation/echo stuff will work fine.
Usually the difference between guitar and bass distortions is a clean blend or a few capacitor changes. So, how about a DIY clean blend pedal?
http://seanm.ca/stomp/bblender.html
That's really just a schematic, though. So prolly not the best starting point. : /
edt-- just realized a schematic is actually what you're looking for. I have a bad habit of not reading things.