Why does it need a tray table? We have a Handysitt at home & out - it folds down flat for travelling & looks quite nice too. Only problem we've ever had is in Chinese restaurants (not sure why just them) as they have chairs with very high backs & you can't hook it on.
Top tip for a booster seat - put a sticker with your mobile number on it - will save you driving on a 40-odd round trip to collect it when you've left behind after a good pub lunch
We have this one as an alternative to a high chair. Very comfy and can be used with or without the tray (tray is quite small though). Bit like a Handysitt, but with the tray. Folds into a carry bag, and is quite light. We use it daily and it is still in good nick. The covers wash in the machine very well (covers being the one disadvantage compared to an all plastic one that can go in a dishwasher).
We looked at the Mee Too (love them in Wagamamas), but you can only use them on tables that are totally solid - something we couldn't guarantee when travelling.