Hi There

When I did mine I used brown packing tape on the board around the area in question, a couple of layers would be fine.

I held the boards in position by inserting packers using slithers of timber I had around.

Turned hulls upside down then carefully pushed some styrofoam cut into long thin wedges to match the profile of the board, then I filled in around the board with the filler mix not too runny and not to thick, the foam stopped the filler from going through too far, and when I tapped out the board, you can break out the foam, even if it leaves a bit in there, it is clean itself out eventually pulling/pushing the boards in/out.

I hope thats sort of a clear description.

Regards