Take a look at some of these accessory pegs. The bolts have a pair of spacers with a conical edge. The conical spacers help tighten the wrap of the clamp on the tubing. If the tubing isn't the right size to make the little spacers work well, see if a few washers under the spacers will help?
Also, try wrapping the tube or lining the clamp with some self-adhering grit strips like one would use on wooden steps/treads to prevent slipping when they are wet? The strips are cheap and if you tear one up in a get-off, it is easy to add another wrap.
"beware the grease mud. for therein lies the skid demon."-memory from an old Honda safety pamphlet