I decided to replace the sawed-off flanges on that salvaged heatsink to make all three use the same mounting arrangement, whatever that might turn out to be.
Nothing particularly fancy about it: two random chunks of aluminum sheet and two thinner strips, sanded to roughen their surfaces, and epoxied into place.
The repaired heatsink is marginally taller than its siblings, but not so anybody will ever notice, and it’s no more off-kilter than they are, either.
A quartet of 5/16-inch lathe bits provided the right spacing to hold the heatsink over its new flanges while the epoxy filled in all the gaps and irregularities. I probably should have paid a bit more attention to squaring things up, but it’s good enough for what it’ll need to do.