More woodwork and metal work later, the "rack" is looking more like something that could work !
Elements are still missing but some devices are on their way :-) I'll be able to install a combined voltmeter/ammeter soon.
The big question is how to attach the variac to the frame but Alex might have found a great solution.
And the other big great news : the variac suffered from bad packaging and the fuse holder broke during delivery but now (after 2 long years) it is repaired ! In part thanks to Alex again :-)
BTW, I'll soon mount a DIN-rail module to display current and voltage. The current sense is inside the module and one wire goes through it :
This simplifies my system because I can power it from the secondary of the isolation transformer while sensing the current that goes to the wall plug. What I still need is an AC voltmeter to measure the output of the variac.