Rog,
My Rosetta is fixed inside my fibreglass pod on the flexwing using a couple of strips of self-adhesive Velcro stuck to the top of the Rosetta, but leaving the removable 'lid' separate. You can attach a velcro 'mounting-pad' to the aircraft bodywork by pre-priming the bodywork first with 'Evo-Stik' and letting it get nearly dry then attach two strips of wide self adhesive Velcro side by side a couple of inches longer than the Rosetta (this makes it easier to find).
Whatever you do, DON'T try to run the PAW upside down as the GPS will be screened from the satellites by it's own PCB. Come to think of it, if as you say the coaming is carbon fibre, you may need to use a remote GPS and antennas in any case as carbon fibre is not RF friendly.
Regards
Peter