Measure the inside dimension of that rear section of box steel, then go to Metals Depot & get them to send You a length 20"+ the length of that section that slips inside/thru', then mount those casters to it. It's no wonder that thing is a tipsy POS, put a couple holes thru' it for bolts/pins that will keep it from trying to slide side-to-side, and You're good. And You can easily 'undo' it if storage space is tight.