Swagman used to make them in the '70's.
Plain flexible plastic/cloth base into which you cut out the filler neck. Straps then passed from oneside to tother under tank and held this base in place on the tank. Two straps per bag then held them to the base. I had one pair over the tank and another over the seat when flitting. Worked well, but as was common with stuff of that era, not waterproof and you could end up with 4 buckets of water slung on the bike!
I still have at least one set, so after this weekend will look them out and try and post details if there is interest.
Cheers, Bill