So ok, I'll go for 1,5 mm aluminium but am not sure
what grades might be available here in Belgium,
not even sure we have the same codification.

== in Europe take AlMgSi1.5, comes close

But the application isn't really critical so I don't care too much.

Allow me the next question: how to finish the panel?
I suppose it is cut to size, then drill/cut for instruments
and fuses and switches and whatever else, deburr,
perhaps sand or polish? then paint, then assemble, then install?

== make an Autocad drawing and take it to the waterjet cutter