I got a thin piece of plywood scrap about 24" x 24".
Cut a square window in the middle.
Snagged a whole box of used continuous printer paper from the garbage atwork.
I hang 2 pieces of paper, still attached to each other, vertically over the window, and secure it to the wood with 4 spring loaded clamps.
The whole contraption is hung in my basement with 1x2s from the ceiling joists.
Only takes about 30 seconds to change paper.
A roll of paper might be better since sometimes the perforation in the middle tears and I have to start over, but hey, you can't beat the price.
Very cheap, and works fine in my basement.

Then I found out how much more valuable bare shaft tuning is. [:@]
But paper is still a good way to get a decent starting point.