Look at Henkels/Loctite UR 8431 and the other 843X series of moisture-cured polyurethanes. I spoke with Styromax in AU several years ago, one of their larger manufacturers, and they were using a similar Henkels product. It was 2 part poly not available in North America. After speaking with Henkels tech for some time they spec'd the UR series of polyurethanes for XPS to FRP. According to Veterosina, an FRP manufacture located in Europe and the US, this is one of the most widely used adhesive types for those type panels.
This product pours out easily and can be hand trowelled or metered and rolled out. The caveat with them is they require pressure during the setting time. We had a 10'x20' vacuum table that we used for our panel construction. I'm sure you get adequate pressure with weighting as well for small runs.
Henkels/Loctite tech support is super helpful.
FYI talk with the folks at Veterosina for FRP, their products are light years ahead of "filon." Plus they offer dozens of weights and finishes.