Find a metal shop in the area. Around here Canfield metals makes large zinc and other metals for the plating industry. I had them make 2x4x18 in block of lead for me to bolt to the bunks. On a 20K lift with triple cables, yours are double, 20' long bunks and the additions above I needed 6 blocks to reliably sink the cradle.
Still need a chain cover, are they still available from Hi-Tide? Geeze I hope I never lose mine. You should also pretty that thing up with motor covers too.