Data from my successful level-2 certification flight, flying a GLR Thunderbolt 38 with a Cesaroni J285-16.  The Parrot altimeter was configured to provide a low-altitude deployment of the main chute at about 750 feet AGL after the motor's ejection charge performed the apogee deployment.  The Parrot, a Beeline transmitter, a deployment battery, and an arm switch fit onto a 38mm diameter avionics sled that is 2.7" long (see right).

The Beeline was mounted on the back side.  The arm switch and the Parrot's on/off button were accessible via airframe holes.