Impromptu solutions:
1. Go to a Harbor Freight, Home Depot, Tractor Supply, etc and get a set of ratcheting tie-down straps at least 1.5 inches in width - wider the better. Toss over the topper, around the slide box, and tighten.
2. Rope. Same fashion but get pool noodles or pipe insulation to cushion at the edge of topper and walls of slide box. If you don't cushion you will get some damage to the topper.
You cannot increase tension to the point of halting wind whip without causing other issues with the topper.