You could try drilling a hole thru the plates and the post and installing a bolt thru the whole assembly or weld a dowel pin to the post to lock in in position after it's drilled thru.
Just assemble it as if you were assembling it to ride, then drill your hole thru everything, once your hole is drilled, just put a bolt thru the whole assembly, or disassemble it and weld a piece of round bar stock that's long enough to go thru the seat post and stick out about 1/4 to 3/8" and weld or braze it into place, this will stop it from shifting side to side when it's running. having a dowel pin on the top and the bottom part of the mount would be even better.
If you choose to use a bolt instead of a dowel pin make sure the hole is the same size as the bolt shank to where it's a snug fit for the bolt to go thru, it don't have to be so tight that you need to hammer the bolt thru, but it should be tight enough to where you feel some resistance pushing the bolt thru to prevent it from wallowing out the hole.
you can always weld the bracket to the seat post for a permanent install if you're sure you won't need to remove the brackets again for any reason.