I own a Fisher F75, but was never too happy with the available covers, so I made my own. I have been fortunate enough that other detectorists have also liked the design. It's very seldom that a hobby can be said to pay for itself.
I hand make these battery covers out of recycled PVC backed material, and being handmade, each cover is just that little bit different from the others. The cover is lightweight, but waterproof, with the seams taped where needed. The battery box cover has a retaining strap just under the back access cover, and small tag covers over the headphone socket and on/off switch.
The cover is available in either green or black, just let me know which you would like.