<< I did this by dry-mounting paper to plexiglass. It worked very well. My
question
now is how should I separate the two without risk of ruining the print
either by
heating or wrinkling ?
>>
Don't use dry mount in the first place. Just wet the paper and then use gum
tape to tape it to a piece of glass; plywood wood work too. It is very
simple. You can check any watercolor painting book if you need more
information. It should be under "stretching the paper."
The only difference between your application and traditional watercolor
painting is that you need to wash the whole thing several times, so you need
to waterproof the tape, which I believe Dick has talked about too.