On Tue, 12 Mar 2019 04:28:26 +0000
Bill Wright wrote:

To hold the fitting use self tappers with heads that fit the
countersink in the fitting. Use over-long self tappers then grind off
the excess length on the other side so it is flush with the
aluminium.

I'd countersink the back and maybe use domed nuts on the front, with
conical washers if necessary to seat them properly on the flange.