I have these Lindam ones from Argos which we just took on holiday and they're great although don't think they'd fit the dt's when they're bigger toddlers (they're 11 months). They're fab and smaller than the other booster-style seats I have at home. But rereading your post, probably not suitable?
I have a fabric one as well from ThatCuteAge (totseat) but haven't used it yet as my two are BLW and without a tray it's just too messy! It's this one, I think it looks great and v.portable, very secure to fix onto a chair unlike another one I had with fabric ties.
Highly recommend Handysitts - pricey but well worth it. My twins are still using theirs at 3 and a half. Brilliant for taking on holiday or to friends as they fold up small enough to fit in the back of a buggy. We were sold on them after a holiday in France - most restaurants only had one highchair.