Heatsinks with a "lip" for mounting components?

What I'm trying to achieve is essentially the same concept as the way transistors are mounted on the external heatsinks in most amp cases. However, I want to use a smaller heatsink, mounted internally. This would be instead of the heatsink and transistor being mounted referenced to the PCB. The intent being to be able to use a larger heatsink and have the heatsink and transistor securely mounted to the case.

Trouble is, I can't find generic heatsinks with this sort of "lip" for mounting. I'm sure I could modify a traditional full-finned heatsink, but I'd worry about not getting a smooth mounting surface.

Does this sort of design have a name?
 

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The heatsinks for these transistors are already mounted internally. I'm looking for a way to increase the size of the heatsink, and also mount the transistor directly to the heatsink/case, as opposed to currently where the heatsink is mounted to the transistor and just supported by the transistor legs.