We don't use those plastic plug any more as they disappear over time!! get a bit of 2 mm thick steel strip wide enough to cover holes, cut the strip in to length's that will fit up in between the rod's or ribs still in the concave grate and then drill one hole through the middle of the steel strip and bolt in the strip that will fit through holes in concave grate then bolt up tight!! (weld in the bolt via the head to the strip!!) Never have to worry about them again!!!! In the concave we used some bolt's and nut's to block up any holes and have not had a worry since!! (took awhile to put in but it's done now!!!! Good job for Guy's like "ROM"!!!! YOM!!!) Rolf