I did a 2 part 3D print to hold the bolts in position while pouring concrete. Seems to have done the trick. It's very secure - the bolts didn't budge at all and I could pick up the entire thing by any point and didn't have to really be careful handling it.
Printed in ABS. I ran out of material while printing the flat plate, but it was close enough to be done, and smooth at the bottom, so didn't bother reprinting it.
I guess the final proof will be when I put on the actual plate - I'll post an update once I have those.
STL files on:
It's specific to the CGX/CGX-L (or similar 3" radius with 5/8" bolts), but it's in OpenSCAD and trivial to modify the dimensions.