Quick and easy...
I took a sheet of thin plywood (the thickness of my backriveting plate., about 4 ft long and 3 ft wide, and cut it into two pieces. I cut out a notch in each of the pieces along the 3 ft long edges, half the size of the backriveting plate, each notch being 1/2 the size of the plate.
Lay the wood on your worktable, put the plate in the "notches", and have at it. I should note that I built two work tables per the plans from the EAA Chapter 1000 web site, putting the tables end-to-end allows me to put the backriveting plate and wood pieces anywhere on the tables with plenty of flexibility and room to slide the project back and forth. It also allows the table tops to remain intact.