I think I've read that the top 2 holes with a little bending/filing/blasphemy can line up, and just snap a little plastic thingus (like the screw hiders from flat-pack furniture) to go between the vestigial bottom leg and the shell.
The Gibraltar works pretty well on my late-50's Slingy. The top two screws fit perfectly into the holes in the drum. The bottom screw doesn't come close to the hole, so I "custom cut" a piece of cardboard to fit under the strainer where the bottom screw would hit the drum. Works fine.
Here’s what I did for an RK shell: an Inde strainer secured with the bottom bolt and a small strap of thin brass over the top slot that took no time to shape and drill. It works very well and is a much nicer (though more expensive than the Gibraltar mod option.