I found them at a supplier of hardware for truck and trailer
fabricators, they have them in plated and stainless. Get the ones
which allow a lock to be inserted and use small carabiner clips
instead and they will never open unexpectedly on you. To avoid the
curved surface issue I epoxied a wedge under each half so the latch
runs straight, then filleted in with more epoxy. I also installed a
'fork' arrangement, a small plate under the catch plate shaped like a
wide fork to capture the opposing clamp, to retain alignment of the
two dolly halves under load. I think this will extend the life of the
hinges as well, I replaced them with thicker truck hardware stainless
hinges.

Get the heaviest clamps you can fit on, don't be tempted by the cute
little ones! Bigger is best! Slip a piece of 'snug' tubing over the
threaded adjusting portion to keep it from rotating back loose when
unlatched.

I'll email a small photo of the trial fitting.

Bob