Rubber Nuts!!! do they come salted ? only joking
have used these before and not over impressed, rivnuts set with the correct tool are far better but the problem with both fixings is not the fixing its the person tightening up the bolt,there are people I would trust to tighten up a bolt and others that would smash a mug stirring it with a spoon!!!!
But like Mr B I prefer braze
regards
John