There are at least two different products available for locking your winch to the bumper, including the Factor 55 product. But they cost $25 + shipping (yeah, I know that's a fraction of the cost of the winch)
With a little looking, I found something cheaper and even stronger.
As the picture (if it attached) shows, you can find a 3/8"-16 x 1 1/2" button head bolt with a tamper-resistant hex fitting, and a 7/32" tamper-resistant hex key.
The bolt was $2.10 at a fastener company and the key was $1 plus shipping, so I spent about $6 total for both!
If you can't find either, the Olander Co. in Sunnyvale, CA sells the bolt as part #37C150BHCA/T, and I found the key through Amazon via Handyman's Toolkit (Ivy Classic 7/32" hex tamper resistant: amazon.com/gp/product/B0052IP4TM/ref=oh_o00_s00_i00_details)
It goes without saying that you can't apply the same amount of torque via the 7/32" key compared to the 9/16" bolt that was replaced, so I only replaced one of the four bolts securing the winch.
I had posted this before, but I didn't have an answer to the question "how hard is the bolt". I have a friend who is a machinist and he took the bolt, and a grade 5 and a grade 8 bolt and had a hardness test run on all 3. The grade 5 and grade 8 fell right within specs on the Rockwell scale, and the button head bolt came in 3 points higher than the grade 8! So the buttonhead bolt is harder than grade 8, and much harder than the grade 5 bolts Warn supplies with the winch.
I know that if you have access to a welder, you can tack weld the bolts to the bumper, but I don't want to face the tough access if I ever had to grind to bolt out. I like this solution.