@smarm mounting the ESC in the mast certainly looks like a good idea with some advantages
- good cooling
- compact build with removable mast (maybe)
However there is one very important point when running a brushless setup: the wires between battery and ESC should be short (maybe not longer than 20cm to 30cm). If they are longer, this must be compensated with capacitors close to the ESC.
Otherwise there is a risk that voltage spikes damage the ESC.
So pay attention where you plan to put the batteries.