One inexpensive option used here in Oz is cutting the tops off plastic 20 lt jerry cans and mounting those with a canvas elasticated cover, all home made. (This resolves the hard/soft pannier debate as these are both.)
An American manufacturer in Evergreen, Colorado makes aluminum boxes and sells spare parts such as the rubber mounting "pucks". Otherwise, obtain 4 circular hard rubber mounts from a rubber supplier, and use those in the corners of the BMW type racks with bolts thru the middle. Another option is to use steel ex army ammunition boxes but they're a bit heavy.
Tim ('91 R100GS-PD)
Tim ('91 R100GSPD)
The only baggage you carry should be in the panniers