The disposable ones are too expensive and the reusable ones are too bulky - my opinion.
We did use our Floppy Seat in the highchairs when DS was small, but it was too big and bulky to use for long - and it was a pain to fold up again too!
I am in the process of designing my own restaurant highchair cover! It's going to be a light weight water resistant material that can be thrown in the washing machine. Because it is a lightweight material, it is not bulky at all - should take up just a tiny bit more space than a deck of cards! (My test model rolled up into a tube that was about the width of a quarter and was about 4 inches long, but I didn't have the strap/seatbelt sewn on that one so that will add a small amount of bulk - but not a lot.)
If you're interested in more info PM me! I'd be glad to send you a picture and make one for you. (once I get my first "real" one done)
Robin