Our method historically has been to include two pieces of C-channel (one at each end) sized to fit over our frame rails, and have a vertical drilled hole in the rail such that a
hitch pin can be dropped through to keep them on.
This year, we used aluminum angle (2", I think) set to go over the top of the frame rail, and used a
clevis pin and
hitch pin clip arrangement.
(Products linked are for shape not size.)
Both methods have nicely met our criteria of tool-free bumper removal and replacement in under 2 minutes and bumper stability during matches. Both methods are also cheap, quick to implement on practice day without disassembling your bumpers, and should work with the frame style you've described assuming your rails are at an appropriate height for attachment and you appropriately size the brackets and pins.