Not sure on the tympan…mine is on a roll and I cut it off square. Not sure if it matters as long as the tympan covers the area you’re printing.
Pressboard is the best packing but it is expensive. Manila file folders work well for me. they come in two thicknesses. If I need a bit more thickness, a sheet or two of smooth paper for the copier goes in between or underneath the folder.You want a fairly hard packing normally.
You are unlikely to need grippers unless you are printing large solids and the paper wants to stick to the form. Unless I’m doing some stenciling I usually leave them off as they get in the way when cleaning. Grippers also have a distressing tendency to smash gauge pins and even type.
I use both 8×12 and 10×15 C&Ps regularly at home and the university.