Does anybody know how to turn those plastic Folgers "cans" into a float for a crab pot?

I can quick answer that in two words ( Hot Glue). You can make holes through the handle area for the string or whatever, then glue around the inside and outside around the sting (cable - whatever). Then put the lid on and hot glue it in place.

Hope this is the answer you wanted.

I do something similar to turn water keel decoys into weighted keel duck decoys.

Bobby is right , his method would be much better.

I don't think that I would use a Folger's can for a float. I have used empty Tide detergent bottles, and also Minute Maid gallon orange juice bottles for floats with good results. You just have to put the lid on tight and tie the rope to the jug handle.