I have a newly constructed 4 bar mech for sale. $120. plus $25 shipping.
Constructed of 1/8" angle aluminum, 1/4" flat bar aluminum.
Operating at 50 PSI, this think has some serious kick!
You'll notice a PVC pipe in the pic, it is a muffler for the exhaust, this thing is quite. No hissing.
The back bar is angle aluminum, so you can mount it in a chair, or wall, and then attach your prop to the front bar. I can just drill mounting holes there or attach a piece of PVC to insert your prop into.
I have this up for auction, but can build more quickly, or just end that one early.
This is a good deal for someone that wants to start in air at a reasonable price.
I'll post a video later today, of it in action. In the meantime, here is what I did with a similar set up.