I haven’t built one so I’m not sure what size you need but here is one
https://www.amazon.com/uxcell-Round-U-Bolt-Length-Washer/dp/B093SRDNVP/ref=sxin_20_pa_sp_phone_search_thematic_sspa?c=ts&content-id=amzn1.sym.3eae31bd-1e34-4d19-88c5-21eedecc026f%3Aamzn1.sym.3eae31bd-1e34-4d19-88c5-21eedecc026f&cv_ct_cx=U-Bolts&keywords=U-Bolts&pd_rd_i=B093SRDNVP&pd_rd_r=3abe60cc-a043-4d66-a0a6-87bf5126f1af&pd_rd_w=l58rT&pd_rd_wg=yTnhq&pf_rd_p=3eae31bd-1e34-4d19-88c5-21eedecc026f&pf_rd_r=0965ZD7MMPW73RYTCA6B&qid=1685278402&s=industrial&sbo=RZvfv%2F%2FHxDF%2BO5021pAnSA%3D%3D&sr=1-15-bc460bb2-6511-44a2-b50d-536026dccc79-spons&ts_id=16409751&psc=1&smid=A1THAZDOWP300U&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzRjFDRzNaTzJXSkpEJmVuY3J5cHRlZElkPUEwMTAzODI0MTRMQjVXMzJYWklZSyZlbmNyeXB0ZWRBZElkPUEwNjQxNTY5MVEzVThVODNGOUw4MSZ3aWRnZXROYW1lPXNwX3Bob25lX3NlYXJjaF90aGVtYXRpYyZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=