I have one that I bought for my Gold Wing but never put it on. It was 4.4 gallons, I believe, and came with interior baffles, painted black. It was to sit on the passenger seat and was bracketed to the passenger hand rails. It was supposed to give you an 11 gallon capacity (with the GW 6.6 gallon capacity), supposedly legal for Iron Butt runs. Since the original GW fuel tank was under the seat this additional fuel tank did not cause much in the way of "top heavy" problems.
I'll go out and get a picture of it tomorrow. Since I no longer have a GW, I probably ought to put it up for sale on ebay or something. Cost me $550 new and has never been used or installed.
It used a gravity feed line (never purchased, but easily available) from it to the top of the GW fuel tank.
Dennis