I am replacing a toilet seat that never sat tight and was already in the property when we bought it and the fittings snapped recently. It is a top fix only pan. The replacement seat came with similar fittings however on fitting it seams the gromits/bungs aren’t tight enough. They seem to be standard size on any site I’ve seen but it appears my hole walls are to shallow or the the expanding bung is the incorrect size as even when screwed in to the bung will pull out the hole 5mm meaning the seat will never fit securely. Are there different expanding gromits as everywhere I have looked online sell standard sizes. Any one have any suggestions.