I routed two 8mm slots, 8mm deep, in what was left of the oak floor board. It would have been better to use a round-nosed 8mm bit, but didn't have one to hand.
Back to the chop-saw to cut to length 35mm.
Rotate through 90Deg, push against the stop and cut again to square.
There you go! A hole in each corner to take a nut and bolt of choice and I have a set of clamps.