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Any flat surface with leather or material of choice glued to it.
I've thought about thick glass as this tends to be very flat, not sure what glue would be required, maybe epoxy or gorilla. A sanding of the leather after glueing may be required with fine sand paper on a flat surface. If you are considering pasted or sprayed flat bed I hear felt is very good.
I believe flat beds are available with magnetic lay ons for different materials, not sure who vendor is. Maybe other members can chime in.
That's a paddle strop in any event, not a bench strop. Handamerican seem to have made one at some point in time.
If you are looking for a cheaper (and, admittedly, far less pretty) alternative to Neil Miller's paddle strops, SRD's modular paddle is a great alternative in my opinion. I have one, and it's great fun. But it's not a bench strop. There are a lot of tips how to build one of these on the internet, though.