The play-doh is soft enough to "bed" a fish in without having to press too hard? You can actually make it soft enough with water? I must admit that I've never tried it, but it seems like it would be hard to get a good seal around the specimen when using play-doh.
Speckled trout,
I don't press the fish into the play-doh.
I place a bed of play-doh into the adjustable mold box. Smooth it out. Then I place the fish in the center of the box on the bedding. Then I build the play-doh up around the fish to the parting line. When I think that the play-doh is at the parting line on the fish, then I spray a little water over the bedding, and smooth it down by hand or finger tip.
Then place your keys into the bedding, mix your mold material and pour.