Best way to electrically and thermally connect a FET to an Aluminum heatsink
Do NOT solder your FET packages to aluminium. The temperature required to activate the flux is higher than the FET packages will be rated for.

It may be permissible to solder a copper intermediate to the alli, then reflow the FETs onto the copper. However, observe the 'time at temperature' figures for the FET packages, and know whether you can heat and cool the mass of the heatsink without subjecting the FETs to 'too long at too hot'. There are lower temperature solders available, which may allow you to dwell longer at soldering temperature.

Bolting is a perfectly sound way to connect electrically and thermally. Try it experimentally. It's so much simpler to do first time, and to rework, that you have to try it before dismissing it.