I made one out of styrene
I Purchased a pack of .100 evergree styrene and testors solvent from a hobbyshop. I cut and glued 4 pieces to make a small rectangle, wide and deep enough to fill entire hole. I then cut ang glued another piece, approx a four inch square on top. I then filled the rectangle part with shreaded lead used for weighting model railroad cars, this prevents the assembly from floating out from any water seepage. Have no problem with this, except for the styrene yellowing, since 96.