With my way too extensive experience of maintaining/replacing toilet seats at a heritage railway, I'd say that they are all utter rubbish. Yes, they will come loose, no, none of them actually fit properly, even the ones that are supposed to be specific to that actual pan.
Wirquin do a seat with a "lock+" thingy which fills that gap between the stupid scrawny bolt, and the big hole in the pan, and I think there's a variation for the top fix ones.
So yes, definitely buy an entire new seat, and expect to have to fiddle with the fittings to get it to actually fit. Sorry I can't be more upbeat about it, but I despair of toilet seat design, and I'm almost scared to go in the local plumber's merchant, as they always ask how many toilet seats I want this time...