I would suggest using a concert horn. The largest they can handle. Might even get one of those tripod like support units that roll so it’s easy to control the horn. Position them so the natural “lean” of the horn points towards the press box.
I would not park them with a sousaphone. If they’re parked, might as well use a real horn and a real tuba would look more professional in a chair.
I don’t know your percussion instrumentation. But if only one snare, I would use a concert snare. It just sounds more appropriate with only one player. Let them play a marching snare in the the stands and pep rallies.