FWIW, my decision for the next time that I have to revisit workholding is that I will have a 3 layer system:
- structure/bottom: original MDF baseplate (sealed with spar urethane or lacquer) with some holes and threaded inserts installed from underneath for adding points to secure from above — a replacement of the original may be easily fashioned by sourcing a piece of MDF, removing the original, clamping it to the blank and cutting it to size and transferring the holes using transfer punches, drills, and countersinks.
- workholding/middle: a threaded insert board no more larger than the working area by the reach of a clamp (but, see below) which has holes in that border area which match up with the threaded insert holes in the bottom layer to secure it (making it the same size as the working area may be simpler), a grid of holes in the working area field for threaded inserts installed for workholding, and additional holes with threaded inserts to secure — discussion of bootstrapping this at: Notes on rapid positions and wasteboard leveling
- spoilboard/top: a sacrificial cutting material the size of the supported working area plus the diameter of the endmill used for surfacing along X, and endmill radius along Y, (with a matching radius at the back corner) which has holes in it to match the threaded insert board for workholding purposes (these may be drilled at need) and holes to secure it to the threaded insert middle layer. Wasteboard Plans with threads
and the middle layer will have new holes for threaded inserts drilled into it and the spoilboard from the top on an as-needed basis using a master file run on the machine — then removed, and the hole drilled through, then enlarged from the other side for the threaded insert — that way I know each insert will get used at least once.