The design looks good and can be used with smaller pieces too.
The issues I’ve had with this sort of non-adjustable jig is that - unless I’ve also cut the 4x5 plate one the nomad - it’s not going to be exactly the right size and will either be too big and not fit into the jig, or too small and pop out.
An alternate sort of jig could be made from an 8x8" bit of Delrin, with a 5.25" x 5.25" pocket cut out of the middle such that the top right corner was in the same position as your top right corner.
You can then sit your stock plate in the recess and butt it up against the top right corner, fixed in place with blue tape and CA glue. The extra space will accommodate slightly bigger stock, and anything smaller will also fit.
When you flip your plate, flip it on the diagonal so that the same corner is put into the top right corner of the jig to ensure it is in the same position. Your design will need to be adjusted so that it cuts the other side the right way around to cater for this rotation.
(edited a few times for clarity and errors and to add a picture :))