Have a small(acre or so) pond with concrete spillway. I use wooden 2x10's that fit in a concrete slot. Water is leaking under the bottom of the board where it meets the spillway bottom. I'm having trouble getting a tight seal. Wondering if u have any ideas for a barrier at the bottom. What about using a thick door gasket material for wood stoves and attaching it to board so that it forms a barrier at bottom(not under the board but on the pond side - would it remain watertight for the summer or would water pass right thru it? Would coating gasket material with tar or something improve water-tightness? How about somew other material? Oh, the board is about 40" long.