You have to divide the board into a rectangular piece and a set of round edged pieces, and position and size them separately to keep the 1/8" radius from scaling.
Looking like this (if I’ve understood the shape of the board correctly):
Make a component of one edge (one along red axis, one along green, if the DC isn’t to remain square) then mirror them to replace the loose geometry on the opposite side.
Clean up any internal edges or faces, build your DC, then hide the edges where the pieces meet.
See this post for a conceptually very similar issue and the solution.
PS. As usual, Dave gets there fastest! He’d finished replying while I was typing. Reminds me of one of the US WW II generals (can’t remember which one) whose recipe for success in battle was “get there firstest with the mostest.”
PPS. I thought it was WW II - maybe Patton? - but a google search says it was Nathan Forrest in the US Civil War, and the quote was later mangled, apparently first in print by the New York Times in the early 20th C.
More accurately, it was “Get there first, with the most men” according to Encyclopaedia Britannica - Nathan Bedford Forrest | Biography & Facts | Britannica.