Bolster backing material - " whaler board "

[abl1111]

I want to upholster my 23' Tsunami. I have 1/4" starboard there now - it's too thin for a backing

I read in the past that there is a material called whaler board because I think Boston Whaler used to sell it. It's like phenolic ( maybe it is phenolic ). I like the qualities of the stuff; inert and rigid.

Anyone have a line on this material ?

I have also read that guys here have used 1/2" starboard - has this stood the test of time ? My 1/4" stuff w/ no bolster material warps in the sun/heat...