Been varnishing the boats after years of abuse and happened to look on this forum for the first time in a long while.....

Did that on a number of kayaks, but with 1/2" Garolite with a layer of glass on the bottom to cover the routed 3/8" groove. Not too much work if one does a bunch at a time. Good thing is they can be glued in after everything is assembled with some mason's line vinyl tubing and backwards clamps. They've withstood the tests of time.