I would buy something like that which was reasonably priced for a 1st gen in a heartbeat.
I made that for the wife prior to getting a top box...........she still hasn't been on this bike. My bike is a Gen III and I used a tall back rest from a 1500 Vulcan Classic that I had left over. Used a torch to heat and bend the back rest to fit the angles and then welded on some plates to bolt the backrest to the same points that the rack attaches to. Painted it flat black and it seems to work well with my test sitting. It's now on the shelf since I purchased the OEM top case.
I'm sure something similar could be fabbed up for a Gen I also.
I'm just getting ready to order the Bestem T-Box with integrated backrest for my 2016 V650LT. I have Givi SR4114 racks already and am hoping that the included plate that comes with the Bestem will attach without too much fuss or needing an additional plate. Any feedback on this cheap trunk? Advice for mounting?