In addition to the aabow links above, check out the Mark 8. You may want to upgrade to latex tubing if the bow part can handle the stress. Also, I appreciate the poetry of this thing being on a cinderblock like an old rusty car.
I was actually looking at the mark 8 thinking the exact same thing. And that's also why I'm here, my bow can handle the stress but I need to know what type of material to use for the plunger tube and what plunger head I could use for it. If Latex Tubing is an upgrade, I'll definitely be looking into it.
On another note: what is the name of that spring that most nerfers use.. It's called something like a K28 or something, I can't remember.