I got the question how I could do a double sided pcb with my single sided exposure box. Well, the trick is to use a L shape piece of PCB. You can drill some guide holes in it, but it is not necessary.
I tape the sheets to the pcb, so you get a “bag” where the pcb can slide in.
For the exposure you have to remove the foil on both sides off the pcb. During the exposure you can place a black piece of paper on top against reflecting light.
Dutch 
