There are a number of solution. Seeing as you have a welder, then welding a plate or nut inside the box is the easiest if the bolts are near the ends. If the bolts are in the middle, then consider using nut inserts (aka rivet nuts):

How to install a rivet nut (rivnut) and the tools you need - YouTube
4mm-8mm THREADED RIVET NUT INSERT SETTING TOOL RIVNUT - PROFESSIONAL | eBay
Small Head Open Steel Grooved Rivet Nuts - Rivnuts - Nutserts - Blind Nut. (uk) | eBay