I like what you are trying to do. I would offer a couple of thoughts.
It is important that the aluminium area under the amplifier is as flat as possible. The thermal transfer is very susceptible (made significantly worse) by any bumps and lumps. Even microscopic ones. (This is an area where many cheap Asian products perform poorly because their casting surface is not post machined). Also, any small rise will create stress when you tighten the two mounting bolts, which can produce internal cracking with multiple thermal cycles. So smooth and flat is good.
Add thermal paste under the amplifiers. This paste acts to fill any microscopic pitting and helps transfer the heat. Thermal paste can be messy.
An example is here which is about $4.00
But you can probably get it from an automotive outlet store.
- The thickness of the aluminium isn’t very important. Beyond about 5mm thick, anything else is just extra weight. A heavier piece of ali will take longer to heat up, but will also retain its heat longer, but it won’t cool any better. Its effectiveness at getting rid of heat is based upon the surface area. For this reason, the use of some fins is often recommended.
I attach an image of the sort of thing I was thinking about. Costs about $30, I think. You will find similar products in Europe from various electronic wholesalers. RS Components or Element14 I think are both available but usually a bit expensive. Digikey or Mouser are usually more competitive.
If you try this link, you will find hundreds of types.
Of course, any air flow you could get past the heatsink would be useful.