That link by ryan87uk was perfect! I picked up a tire that the guys at the shop told me should be about 600 or so. Well I weighed it on a pallet jack scale and it was only 470. So I’ve bolted 4 x 25 plates on the inside to make it 570 and will be adding 4 more bolts so I can load it to whatever I want. I just used 3/4" x 4" bolts, nuts and washers from Home Depot.
The bolt may seem too short but stand the tire up and roll it onto the head of the bolt. This will give you more thread to work with and will press the head of the bolt into the tire so you don’t have to worry about rubbing your arms on it.