Taking the handle off
A piece of 1/2" particle board (osb, flake board, whatever). Cut a 3'x4' pice from a 4'x8' piece and used liquid nails
to glue it to the top. Used ratcheting straps to help hold the wood to set glue.
Took the inside lid panel off and installed a sythetic wood piece. i then ran bolts from the top of the 1/2 particle board to the bottom of this piece to add some stability and help the glue set.