Have you considered using a mercury switch to determine and correct the orientation of the plane ? Its quite simple, a small capsule (think of your standard antibiotic size pill) filled with mercury. One contact on the bottom, where the mercury sits when flat, one contact on either side. This can then be used to move a flap or rudder whenever the mercury makes contact (ie. whenever the plan pitches or rolls, depending on which way you install the switch).
Lightweight and cheap. Mercury freezes at -38 degrees C, so this may be an obstacle depending on how high you intend to go.
Anyway, implementing such a switch could enable you to maintain a steady flight path of 45 deg, or even allow you to start at horizontal on release, can correct to vertical over a few metres, avoiding the ballon then moving to a vertical path.