I have done it two way's. First I drill a hole about 2 inches up from the bottom to tie your rigging line to. Then you set the shell down on a sheet of 1 inch styrafoam & trace it out. Next cut it out. Then place some glue on the edge of the cut out styrafoam & place in side of the decoy. Let set & dry. The other way is to get some of the pipe insulation that has a cut down the center. Spread the pipe insulation apart & place along the bottom edge of shell decoy. These are just some way's that I have found. I am sure there could be some better was. Hope this was some help.