Here's an upside to a cheap frame...feeling free to change it!
I added an Avid BB disc in about an evening.
I mounted the wheel with the caliper clamped in place by a wad of inner tube bands, pivoting it into place carrying a cardboard bracket template.
After trial and error, I made a tab that fit.
The same method was used to position the tab for two tack welds on the outside before running a bead on the inside of the leg. A dab of close enuff paint sealed the deal. Yes, it stops quite well!