OK, thanks to everyone. Looks like I have good options and can just figure it out when I board. Putting it in the coat closet is not a bad solution—-the flight attendant offered this on our last flight 2 weeks ago, when my husband carried on a 22” x 20” x 3” plywood box that contained a painting we were giving to our daughter. Checking in, I see I have been moved from 1F to 3F, so it may be overhead bin or the coat closet.
I did look at the mailing tube option, but the one we have (4” diameter) won’t hold the model kit.