A pressing board is a tool used mostly in sewing to iron oddly shaped and inaccessible corners.
For some more info on pressing boards see
Step 1: Cut Out the Pieces
I cut them out of an oak board I had in the garage. You could scavenge some old furniture, but keep in mind that you would need to get off all the old finish since you'd use a hot iron and some possibly white fabric on it...
The holes at the head end of the screw are drilled a touch oversize so the screws can pull the boards nice and snug rather than binding in both boards.
I uploaded a .pdf with the pattern I made and used and you can tweak it of just make your own. It doesn't require a lot of precision, but I wanted it so that with my 3/4" thick board the whole thing still ended up being as the same size on all 4 small sides, and so that the three prongs at one end would line up so it can stand on end. Maybe you want something slightly different?
I used boards that were just under 6" wide so I scaled my drawing to make good use of the material. You could probably go an inch smaller or as much larger as you want and it would still work.
Step 2: Sand and Sand Again
Sanded it smooth and rounded over the corners. It has to be smooth enough so fabric doesn't catch.
I started with really rough lumber so I actually had to run it through the thickness planer and over the belt sander first, but if you get lumber at the lumber store you likely wont have to do that.
For sanding I used 100, 180 and 240 grit sandpaper, going with the grain where possible. I used a sanding block for the flat and convex parts - if you usually just sand with paper in your hand I highly recommend trying a sanding block sometime. You don't need to buy one for trying, just pull some sand paper around a piece of wood and sand with that.
I used sandpaper around a piece of pipe for the inside corners, and sandpaper on its own to get the corners and everything else nice and smooth.
Because I wanted it really smooth I rinsed the wood off with some water to stand up all the grain, then let it dry and fine-sanded it again.
Step 3: Get Those Screws Ready
Because I don't want light fabric to catch on them I recessed the screws in the wood. I also smoothed off all the edges in case there were any burrs raised in manufacturing.
I twisted them in the grinder and them against the wire wheel while holding the sharp end in my cordless drill. You could just run them in the drill against a piece of fine sandpaper (careful how you hold the sandpaper so you don't slice yourself...).
Of you could just run the screw heads over the sandpaper by hand. That is less fun, though. ;-)