Get a cheap feeler gauge. Determine exactly what thickness to use. Cut a piece that fits into the hook. Tin hook and one side of shim with 95/5 or 50/50 solder. Lead free solder is junk. You can use any heat source you're comfortable with. Fit shim into hook and hold it there with a piece of appropriate sized drill rod painted with something so solder won't stick to it. Various products for this. When everything is set up right, drill rod is clamped with C-clamp or whatever, things look good, etc. just heat the drill rod enough to melt the solder, let cool, job done. Just make sure to clean oil/dirt off hook and shim and use a good flux suitable for steel.
nial