We have used the "First Step portable booster seat" for all our vacations with babies. My oldest is 9 and we started using one when he was 6 mos. (we are on our 3rd one - LOL)
Like, summerluvv said, they are about $20. and well worth it. It attaches to any chair. You will use it this vacation and probably for a few years. Once the baby outgrows the highchair, we use it with a kitchen chair and pull our toddle right up to the table with us.
First Step makes 2 kinds of portables, the one where the sides(arms) fold in is better and bigger, it works better as the baby gets bigger. The other kind, where the tray swings out is kinda smaller and not a good.
An alternative is to feed the baby in the stroller.
Good luck!