Coolant bottle situation is resolved,and in clever fashion if I do say so myself!
After looking around at other bike's bottles,I came to the realization that Ill probably never find one thin enough to fit in the tail section.So I started looking at the stock bottle and other locations around the bike.
Turns out,there is a perfect spot up front,which is occupied by the horn.The space is 10 times bigger than what the little horn needs,and its wide open.Coincidentally,its also about the same shape as the coolant bottle,+1,and just happens to be very close to the thermostat/rad.cap housing, +2.And,using a small spacer,the horn bracket lines up with one of the mounting holes in the bottle,+3......AND.....when in place,the vaguely usable coolant level window is clearly visible from between the frame tubes,+4!!!!
So,I moved the tiny little aftermarket horn to one of the mounting tabs for the cooling fan.No problems there,the wiring reaches and its tucked nicely in place.Then,I had to cut about 1" off the top of the coolant bottle for top clearance.It would have fit without the cutting,but I never would have been able to get the cap off when in place.To regain the locking lip for the lid,I used my trusty heatgun to soften and enlarge the top edge of the bottle,so it locks into the cap.
The bottle now handily fills in a space that is otherwise wide-open and in plain site, from the left side of the bike.It looks like it belongs there,sitting at the same angle as the front frame tube and only a B4 owner would be able to tell it doesnt belong there.I can add coolant with a thin funnel or piece of tubing,not that you ever have to do that much anyways.