Pannier frames/stiffeners?

[Dell Phinus]

Hi,

Could someone describe pannier "frames" or stiffeners to me? Are they a flat "board" (would
plastic cutting boards work), the same size as the back of the pannier? Or just strips than run
the width of the bag where the hooks/straps attach? Are there sides of a bottom to them? And how
is the bag attached to the frame, and the frmae to the mounting system?

I'm toying with building a set, from surplus Musette bags, and Arkel mounting hardware (already
ordered). I'm just looking for something to use on day trips with the family (lunch/snacks/clothes
for 4), and something to play engineer with .

Thanks! DP