I came up with this advanced fixture only after seeing the weaknesses of my original design and contemplating how to solve those issues over several months. Components of the final solution came to me slowly.
My first realization was that bolting the jig into the TOP of the frame members would be far easier than bolting into the inner side of them. I made a few small prototype angle clamping jigs to prove my assumption, and they are indeed easy and quick to install.
Using the front rail (rather than both side rails) to clamp the fixture to was my second "aha!" moment. The front rail is always parallel to the gantry, so using it would eliminate any need to "square" this fixture when installing it.
The support plate of the fixture has an array of rectangular holes to pass c-clamps through for clamping parts to it. Other "assist" brackets can be attached to the support plate for unique clamping needs. I like using c-clamps through the plate, and will borrow this strategy for future CNC bed and fixture configurations.