I'm going to mention a few things that I have mentioned before:
I used translucent Polyester paper.? I don't have a source right now. Toner adheres very well and is opaque.
I did use targets and alignment holes at the edge.? I used thumbtacks to align the masks on the board.
I did spin coat the resist.? The alignment holes do create a problem with the spin coating because they were made before the spin-coat not after.
I just used the surface tension of water to hold the polyester paper in place with the alignment Thumb tacks for registration.
I would image that there is room for improvement:? For instance using a sheet of acrylic and a PC drill to create the alignment holes and drilling under a safelight and doing the making of the alignment holes after coating the boards.? You could probably find some machined pins for alignment too.
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