Thanks for the responses.

I've cut a hole in the kayak. Mostly following the fitstorm model. I've
cut a hole centered on the compass mount area for a 1.5 inch PVC pipe
sleeve.

Instead of connecting to a shelf as in this model, it continues to the
bottom and is embedded in a square block of minicell foam glued to the
bottom with barge glue. The bottom end is capped and glued inside the
foam.

I still need to find the Elastomeric Marine Sealant to seal around the
pipe sticking up, and trim it cose to the deck of the kayak.

I'll try to publish pictures and steps once I've gotten through the
rest of the process.