On my RV, they originally came from B&B Moulders but they are out of stock. My lower corners were 5.25 square. Tried to find for a year or better and ended up just 3D printing one. Looking today, I found some that appear to work from ICON (wouldn't you know it) so you might want to look on thier site for a mataching size. Good luck.
https://www.icondirect.com/gulf-stre...-cap-set-of-4/
Good luck
Ron