I don't know that something like this needs to be welded steel (though I'm thinking Beseler MXT, your enlarger may be 5x the weight?) I'll eventually wall-mount my MXT, and my plan is bracing 3/4 plywood sheet to the wall studs, and building sort of a screws & glue plywood "box" that emulates the stamped-steel Beseler mounting kit, and gets the head a bit further from the wall. I think I can guarantee that if done properly (and with a similar concept for the top of the frame), it'll be as rock-solid as any steel piece could be. YMMV of course, but in my mind this can be a table-saw-and screws project, not necessarily a welding gig.