I had an issue with trying to get my clamps to fit on that engine guard without slipping. Seems I couldn't clamp them down tight enough. I went to my local hardware store and bought a strip of grip/friction tape that was the appropriate width. In my case they carried several widths on rolls so that you could purchase it by the foot. It's the same stuff that kids use on skateboards and that is used on steps for non skid applications.
Find the sweet spot that you want to put your clamps on your engine guard and put some on the inside of the bar. It doesn't take much, less than half way around is more than enough. After installation whichever highway peg clamps you choose will work as well as putting them on a regular engine guard. In other words it's not the clamps, It's that cool looking crash bar that gives you trouble on this application.
Hope this helps.