First a Jig for pattern sanding. The inlay is made from a friend machinist.
 

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For thickness sanding:
 

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This a comple cover over the jet. Just put it over and use the jet as thickness sander. I made a cylinder out of aluminium which is not so soft as the original spindles.

 

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If someone has also good jigs for the jet. Please do not wait and show them.

Nice, I have a Jet J BOSS spindle sander.can you give more details on your aluminum spindle?

@Roberfishes
I drilled a hole (16mm) through a aluminium cylinder and put this on the spindle screw. ( i have a little lathe to this).
The sanding paper is diagonally fixed on this spindle with some tape on the ends. Works great. The images shows my first test with a plastic cylinder.