We bought the Tomy Freestyle Three-in-one booster seat so that we had somewhere to put our 18month old daughter when we were out and about, or visiting friends where there was no highchair available.
This booster seat uses the carry handle as the strap that attaches round the chair.
I don't think our daughter is wigglier than the average child, but she was rocking it from side to side, and even though the harness was as tight as possible she could easily have climbed, or tipped herself out of it.
I think part of the problem is the booster part of the seat being non-rigid making it easier to sway from side to side. Unfortunately we'll be heading out to find a rigid yet portable booster seat for our upcoming holiday.
On the plus side it does have space for nappies and wipes, and a net mesh to hold a drink, so you could travel without a changing bag if you were going out only for a short time.