'm still murky on the red line dividing between panelboard and UL508a and listed electrical box with stuff in it.
Panelboard: listed under UL 67, contains circuit breakers, bus bar, circuit breaker accessories. You technically cannot put other things into a UL67 listed panelboard that are not already listed to be put in there.
UL508A is a program by which a panel shop can use a "procedure" approved by UL to assemble an Industrial Control Panels (ICPs) using
other listed products and apply an overall UL listing to the assembly. There is a table (SA1.1) in the UL508A standard that lists all of the
other UL standards that are valid to use in a 508A listed assembly.
UL67 is NOT one of them. So you cannot, for example, use the guts out of a UL67 listed panelboard as a means of distributing power to a bunch of circuit breakers that then feed motor starters in the same panel, because the UL67 listing is not acceptable to a UL 508A procedure. On a case by case basis, if you happen to have a UL inspector handy when you want to do something like that, they may allow it, but without that, you could risk losing your listing status. But for sure, you cannot use UL508A to built a UL67 listed panelboard. You CAN however, find UL508 listed bus bar assemblies and bus mounting accessories for attaching circuit breakers to them, and build up the EQUIVALENT to a panelboard from that. It will however likely cost you 3-4x what a factory panelboard would cost...
Dead fronts in a UL508A panel would technically have to be listed under UL50 as part of an enclosure assembly. Most of the enclosure manufacturers have those options available. But would a UL inspector pull your license for using a custom made piece of sheet metal for that purpose? I doubt it. As to gauge, I used to see specs that called for "minimum 14ga flat mild steel, or lighter gauge formed and fabricated to have equivalent rigidify to 14ga". Most of the ones we used were 16ga with a bent lip to stiffen it.
