I did something similar (many) years ago - I had a friend weld some large washers on (also large) eyebolts, which I then bolted to the bumper on my MGB. I drilled holes around 5/8" - eyebolt diameter - on the top of the bumper, spaced the same distance apart as my fork legs, then added smaller eyebolts on the extreme "corners" of the bumper. I removed the front wheel on my '66 Triumph TR6 and slid the axle through the eyebolts to attach the bike to the bumper, then ran (tight) lines from the top of the forks to the more widely-spaced smaller eyebolts to keep the bike from moving. It actually worked great - that's how I moved to Berkeley (from SoCal) for college, and I also did it with my Rickman Weslake several years later. I just had to come up with a reasonable story to explain the four holes in my bumper to the guy that bought my MG.