Is there a protective pouch for a bodypack style wireless microphone transmitter that can be attached to a guitar strap?
Here are companies that offer protective pouches for a bodypack style wireless microphone transmitter specifically designed to attach to a guitar strap:
Dunlop Guitar Products - Guitar Strap Pouch model SA10
Fabric bag with Velcro and elastic loops to attach to guitar strap.
Levy’s Leathers Ltd. - Wireless Transmitter Holder model MM4
Pouch with leather straps, Velcro closure, made to attach to guitar strap.
One additional idea: you may find a suitable cell phone protective pouch that fits a wireless transmitter. Take your wireless transmitter to a local cell phone store and try different pouches for size, style, and suitability.