My old National Semiconductor data book shows the IRF510 chip size....
98 thou x 87 thou
2.5mm x 2.2mm
The heat-generating area is a bit smaller than that.
Drain is bonded to the metal tab directly while the source
and the gate are attached with flying leads that conduct very little
That chip is a tiny heater bonded to a far bigger slab of metal, which in
turn is molded to the IRF510's black epoxy.
That 3.5 C/W junction-to-tab heat spec is likely due to the small chip
size. The much larger contact area of metal tab -to- heatsink likely
means the tab runs just a bit hotter than the heatsink under it.
We used to rely on the black anodizing on the heat sink as an insulator.
Although the aluminum oxide is very thin, it is a remarkably tough insulator.
This was for 60W Class AB audio, where two transistors bore the heat load.
Not saying you should try this because I don't know the quality of that
heatsink extrusion - a little scratch can cause mayhem. As always,
follow Hans' excellent instructions.