I have the enclosure depicted below, made from sheet metal. A magnet says: steel (well Iron to some extent anyway

)
From some Chinese seller on amazon, IIRC. Laying around for a while.
Dimensions: 24 x 19 x 11 cm³, sheet metal is 0.75mm thick.
Well not quite "metal case", because, as you can see, it is held together by bent metal sheets screwed to the front/back white plastic frames.
The outer most ones have nuts, the center ones are self-tappers... (which could be corrected).
Now I also have a bunch of same-type PCBs from china ebay (each with 8 relays, so some weight) with a M3 hole in each of its 4 corners, that, if I stack 4 of them on top of each other with maybe spacer tubes on long M3 screws, two such stacks next to each other, it will roughly take one half of the case interior, and the other half has space for PSU and control electronics (and a bit for front panel controls).
(I feel the metal is too thin and space too low to put a AC mains PSU with its socket in there, given the forces of plug wiggling - so the mains part will be external)
I'm just not sure how, and where / to which parts of the case exactly one would actually fasten this to.
How would I get those stacks of PCBs in there - if it's at all advisable for this somewhat flimsy seeming case?
Although I guess I could bolt a few strips of sheet metal in some places to make it a bit more bending-resistant. Especially where screws are going through.
Say I have 4 vertical M3 screws stacking 4 PCBs: Do I just drill 4 corresponding holes in the bottom of the case, put some bigger reinforcement washers on both sides of the bottom sheet metal and fasten it with nuts?
Or maybe strips of sheet metal from one hole to another as better reinforcement?
I guess the 4 screws going through all PCBs of a stack could reach high up through the top sheet metal, and after closing, add nuts on top, which make it more stable, but it doesn't seem aesthetically favorable having screw ends stick upwards out of the box. Mounting this with screw heads on top would seem more difficult, especially with regards to connecting things when the case is... close and its contents inaccessible - unless *everything* is top-mounted, dangling in this flimsy, bent metal sheet, probably not fun to assemble.
Any suggestions?



Stack 4 of those suckers vertically, making two stacks then occupying the left half of the enclosure.
