Messy but fast. I've had pretty good luck using Loctite 3108. It doesn't make for the nicest cured surface but thermal shrinkage and deformation is pretty minimal.
What you need:
Draw or print the channel shape that you want. Check to see if your epoxy is negative or positive. Take a slide and make two little spacers for each end roughly as tall as the thickness of the slide. This will be the depth of your channel. Lay your transparency mask on top and expose with the UV source until mostly cured. "Develop" your device by washing off the uncured epoxy. Top with a plastic slide (with entry/exit holes cut out) and cure again to seal.