Heavy rain and saturating windy mist is a problem for current e-calls. I hunt true rain forest at times, places with up toward 200 inches of rain per year. I wish calling units were totally waterproof but it would add a steep price increase. Somebody else will have to answer for a FoxPro specific cover, but even such bags have some kinds of holes or seams. My comments are more general, about using any e-call in rain.
We transport the e-call inside a sealed Ziploc to the stand and carry several plastic bags to protect the call on stand from water, from trash bags to Safeway grocery bags to Ziplocs. I've called in the wet with K-Mart boom boxes, home made e-callers, a souped up Cass Creek and an M1 Bandit.
My usual method is to place a bag of appropriate size and color above the caller like a small roof or umbrella. Sometimes I slide an open large Ziploc over the unit, sort of like a tent, open bottom skirting down around the e-call. Wind can be a problem. I have a small sack of clothes pins in my pack to anchor a bag if needed. I always place a closed cell pad under each elecronic item, cut and carried along for that purpose.
In forest I can often put the caller under something like heavy overhanging limbs, under a log that is well above the ground, etc. I have propped a big slab of bark like a roof over a caller also. I've never sealed the M1 inside a plastic bag because of concern for sound muffling and also because of decoy attachments, but that wouild work on a FoxPro, M1 or any small wireless unit.
I partly chose a Bandit because of the waterproof Pelican case. Of course it has holes drilled in the case for switches etc. I've never had trouble with it in the rain but I protect it as mentioned.
The Sidewinder decoy has a waterproof plug in the Pelican case where the motion drive shaft comes out. I wrap the case in a plastic bag, skirting tight around the drive shaft, but the decoy top has to be exposed enough for a critter to see it or it is useless. The motion tail sheds water better than I expected and does not get too heavy to work.
I have considered carrying a child's umbrella to place over the call but haven't yet. I hunt more in rain than without.
