Getting the two sides of an exposure mask to align well when making a two sided PCB is always a pain. With a simple cardboard jig it becomes easier. Quite surprisingly I have not seen this idea anywhere. The usual advise is to make a pouch of the two sides of the mask, but that does not help much when you need to turn the pcb around to expose the second side.
What you need is a piece of cardboard that is a little thinner than the pcb that you intend to use. I am using 1.5mm pcb’s and I used 1mm thick cardboard from a box that I had around.
Tape one side of the mask to the jig, matching the image to the opening. Then align carefully the other mask to the other side of the jig and tape it to the correct position. The pcb goes in between the masks and the right angled slot in the cardboard makes accurate positioning easy.
Below is a photo of an etched and drilled double sided pc. The trimmer pot holes (green) are slightly misaligned, but the stereo socket holes (blue) are very well aligned. I was not careful enough when aligning the masks in the jig, but the result was still good enough.