If you don't find a press, you can also consider the following:
Canson Baryta, which is not RC paper a the Lasal. The RC is the issue you are having with a spray mount, as the RC will not really ever allow the spray mount to bind. The Canson is Alpha Celluose and excellent paper. As Gator has a rough surface, this would allow for a very good mount. I would try the Elmers Acid Free or Standard 3M Super 77. Since Gator is not acid free, anything but, the spray not being acid free should not matter much.
As for M Muck, I don't think it will work on RC, just won't hold long term. It needs a porous surface, like Canvas. Also M Muck is very viscous and Lasal 300 W may have some issues laying down evenly on it.
Paul