Maybe buy two Bubble Bums and duct-tape them together for more height? (Though nominally the seat is supposed to be about 4.5" high when inflated, which is close to your desired boost.) Then, after the concert you can keep one in your car and one at home. They don't have a maximum weight limit - the site says it is made with the same materials and tech as life rafts. It's not squishy like a plastic pool toy, it's got memory foam so even when deflated it meets safety standards. It looks like Target sells the BB so you could go to a store if you've one near you and check it out. I've used it myself on occasion and I also weigh quite a bit more than a child.
(NB. I'm not related to Bubble Bum, we just have found their product very practical and versatile!)
Another option - we also used this booster when my kids were little. It's bulky but not super-heavy, and it comes with its own bag with a handle. You could try turning it upside down and sitting on the flat part. It's made from very dense foam so it can't deflate.
Cooshie Boster