When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Assuming that you're also using new bolts, the exact size and thread pitch isn't critical. Either 8mm or 5/16 should be fine. Just get 1" long bolts to match.
Make sure the u-nuts will clip onto 1/8" thick metal.