How about a full size dipole with the transmitter hanging in the middle?
Usually the transmitters are 2m or 70cm I think. Never done a balloon myself, but always wanted to fly a kite with a full size H.F. vertical antenna.
Hmmm, this gets me wondering, should I make a box kite and fly it at the beach, it's always windy enough there & plenty of room for groundplane wires.
Be prepared to lose some kit on the trip down, terminal velocity sometimes has that effect...