What (cheap) material could I use for a squirrel baffle atop a bird feeder?
Grease. Recommend food grade.

The second material I'd use is a small bb gun, pump, so that you aren't killing them. Just enough to scare them off.

Honestly though you're going to be hard pressed to find something big enough to stop them from getting there. Whatever you do they'll figure out how to get around- otherwise a single piece of sheetmetal that's 2x longer than their body in radius (and they CAN stretch) should make it impossible to swing under.

That'll also turn it into a nightmare in any wind...

You can make it harder, and make it funnier with food grade grease, but otherwise...