Moldable/pourable materials that dry hard?

My plan is to attach the Starboard platform to the rail with four threaded inserts and some thumb screws/knobs, then bolt the Scotty base to the Starboard "permanently". That way I can just remove the DR itself for surf launches and/or travel to and from the beach, and leave the Starboard and Scotty base in place. Or I could just take the whole contraption off at the end of each day...

With four widely spread attachment points (each with sturdy backing material) I don't think I'll need to use the cup holder. That's the beauty of the Outback; we have a nice wide, flat(ish) gunwales to work with.

My plan is to attach the Starboard platform to the rail with four threaded inserts and some thumb screws/knobs, then bolt the Scotty base to the Starboard "permanently". That way I can just remove the DR itself for surf launches and/or travel to and from the beach, and leave the Starboard and Scotty base in place. Or I could just take the whole contraption off at the end of each day...

With four widely spread attachment points (each with sturdy backing material) I don't think I'll need to use the cup holder. That's the beauty of the Outback; we have a nice wide, flat(ish) gunwales to work with.

That's a great plan. Salmon will be jumping in your boat just to take a look at it...

Ha! That's the idea.

Don't think it would stick.  Hard to find stuff that really sticks to the hull.  Other alternative, though quite a bit more expensive, is West System resin with a bunch of Collodial Silica thickening agent (about the consistency of peanut butter).  I'd go for the epoxy putty.

I thought west system epoxy didn't work on polyethylene.

They actually make a version for kayaks.  In general, with proper prep it will work.  His intended use listed above didn't require it to adhere.  Actually better that it doesn't for his use.