Nettie, we have a Cooshee hybak, it's made of the same sort of squishy stuff as a bumbo but designed to be used as a booster on a seat. It is quite grippy, we've never had any problems with slippage, but it doesn't have any straps so only use if DS doesn't need actual 'restraining', kwim?
He probably 'needs' restraining, but doesn't want to be restrained. So we do want something that he can use to get some height, but not strap him into - otherwise we'd just use the highchair!

Thanks everyone for your suggestions. If you have any others that don't require him to be strapped in, please let me know!