I think it could work well - I understand your goal of being capable of adjustment and dismantling. I think it's for you to decide if the effort involved is warranted, not forgetting any internal weld seam on the box-section. Not sure about the 1mm rebate - so the plate this bolts to would not be in contact with the surface of the plug?, I think that's a wrong solution - you want that plug to present a mating surface, even stand proud of the box-section (proud enough to allow a weld bead to be <cough> ground into place.

Feels like a lot of work if you need to do a lot of these. I'm lazy though, and hate to spend any more time behind a tig or burn more argon if I don't need - but would instinctively go for the end cap that Jazz describes.