I have some soft sport bike bags but they have a strap that I connected to the passenger foot peg to stop them from flopping around.
You could make some supports. You'd need is a drill, hack saw, some small nuts & bolts, and some metal bits. For the metal, some flat stock metal pieces that you can pick up at any good hardware store will do the job.
Here's a couple of pics from ebay to get a design idea: You can see there are two holes at the top that you'd bolt to the bike (maybe under the turn signal or grab bar bolt and to the shock bolt). The piece at the bottom would be "hinged" to the two vertical supports by a couple of small nuts and bolts with lock washers or maybe "nylock" nuts (these are nuts with nylon in them that won't vibrate loose, they don't have to be super tight to stay put).
Good luck with your bike. I'm glad you got her going. $1100 for a running, ride-able bike is a great deal. Most of us who have picked up 79-82 CB-750s have been hit with a bad alternator rotor that held us up from the get go, sounds like you got a good one.
Ride safe and be alert.
89 VN 750A - Given to son-in-law
79 CB 750K-sold 3 May 21
78 CB 750K
77 CB 750K
77 GL 1000 x 2
77 CB 550F
Holton, KS, US
Locked
This would have been a great shop class project.)
