Fin Box/Plug Material

I’m refurbishing a prone Paddleboard and am replacing the fin box, but want to provide additional support/stiffness to the box by first installing a block of some material (wood, plastic, etc) that the box would be installed into. I’ve used a tropical wood in the past but found it to be too heavy. So I was thinking of using a plastic material such as used to manufacture fin plugs & boxes because it’s light, sandable, resistant to water absorption, and will bond with resin. Can anyone tell me what kind of plastic is used to make fin/skeg boxes or plugs? Seems like this is the stuff I want. For a 10.5 in box I’d be looking to get it in a 4"x1"x12" block, approximately. Thanks

Fiber filled Lexan.       You’re better off sticking with wood.         Poplar, Ash, Birch, etc.

A couple strips of wood routered into a slot either side of box and running past about an inch at each end would do a lot of good, and add very little weight. John Mellor has posted pics in the past, might be able to find photos with a search.

Watch out with wood blocks for post lamination plugs (like FCS). Small cracks around the box can easily lead to water intrusion into the wood.

HD foam inserts have been used for years in sailboards and work very well.

I 2nd. the High-density foam blocks.

Easy peasy.


-HD PU (tow) foam insert for Gearbox & a homemade Divinycell surround for a Bahne box and the wooden fixture for routing it into a board.