There are
two plastic storage trays under the R65's seat.
The forward tray has a
snout on the front that is captured within the open end of the upper chassis backbone.The rear of the tray has an extension that sits on a chassis cross member. There should be no contact with the battery.
The aft storage compartment attaches to the rear frame hoop/fender and is shaped to fit under the seat's tail piece. The first photo is of my 1981 R65.

The storage compartments are closed off to weather by dense foam pads glued to the seat pan. I made new foam bits using a mouse pad cut to fit. The foamy bits are shown on my 1983 R65.
BTW, the large diameter back bone tube is a fine place to carry spares and tools. Bob's BMW sells a way cool custom bag that slips in. The tool tray keeps things from sliding out. A good place to carry extra ammo, I find.

I searched eBay for a tool tray to go in my 1978 R100S and discovered the junk on flea bay was priced higher than a new tray from Bob's BMW. Shop wisely, Grasshopper.
http://www.bobsbmw.com/There are attach points on the top rail of the sub frame for seat attachment. J-hook hinges, pins, and holes for same.