A dipole is really good and simple and a simple ground plane even better. Simple dipole has your radiating element in one direction and a ground wire in the opposite direction. If you want a bit more range, 5/8 wave will slightly squash the radiation envelope.
Did you see this?
From the sites I’d found, for 915MHz 7.8cm was the desired radiating antenna length. For 902.3MHz single channel devices I got 7.9cm.