[Craig Sawyers]

The problem with thermal pads is that they require an absolutely flat surface, and surface finish at least as good as that of the transistor. Extrusions, like the angle, are not good enough, and finned heatsinks in particularly are banana shaped.

I flatten mine laboriously on a piece of plate glass with belt sander abrasive strip stuck to it.

Then I use mica insulators and heat sink compound.

Anyway you get two bad interfaces with angle - thermal pad to a wonky surface on the angle, and then angle to a banana shaped heatsink.

Put a straight edge (AKA decent steel rule - Rabone, Moore & Wright or similar) on a finned heatsink and look at it up to the light.

So either do the abrasive trick and spend an hour of fun and games, or sweet talk someone on the list with a milling machine, or a local friendly machine shop to machine the back of the heatsink flat with a large diameter tool and slow feed.

Craig