photopolymer and exposure unit

Although, I am not familiar with your machine. You could build a frame with rubber-ish material sheet, stretched on like a canvas for painting. Then connect a vacuum pump to it.

Or you could simply put an extra thick padding on the lid of the machine and install latch locks for giving it some tension.

A supplier of the plates also sells Kreen. I have never purchased Kreen though.

Kreen is a UV-transparent plastic sheet used as the top layer of the vacuum table on professional exposure units. Boxcar sells it (or did last I checked).

It’s cut to the full size of the table (covering all the vacuum area) and taped to one edge to make a flap you lift back to expose the table surface. You place your un-exposed plate and negative on the table, smooth the Kreen over that, then turn on the vacuum pump. This holds everything together with many thousands of pounds of force (nearly the full weight of the atmosphere above the table) and is much more even pressure than using mechanical clamping.

Theoretically, you could drill a series of small holes in the lid of your unit, add a thin, air-tight chamber to the top of it, and attach a vacuum pump to that chamber to turn the lid into a vacuum table that would probably work decently well. I’ve thought about picking up one of these cheap exposure units and modifying it in this very way.
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Michael Hurley
Titivilus Press
Memphis, TN