Are any of your seven boosters nbb's? Are they all high backs that do not turn into a no-back? (Is that what you meant by "not convertible"?)
We bought two harmony youth boosters for travel because of an airplane trip. The two children (who were using them at the destination) each carried them on as their "personal item" and could also still have a carry-on (their backpack with their stuff to play with, a change of clothes, etc.)
The boosters fit in the overhead bin easily. They even stack to save space. They only cost about $13 each at Walmart. Then they stowed their backpacks under the seat in front of them.
We already had a Turbobooster we could have brought, but the two youth boosters together weighed the same or less as one no-back turbo.
We do have a Bubblebum also. It does pack smaller but if cost is a strong factor I would not hesitate to go with the harmony boosters. They fit lots of kids well and make a great spare seat for friends, to keep in grandma's trunk, whatever.