Pretty getto just tap a punch into your bars all over where (AND ONLY WHERE)the clamps go. You can also buy a self knurler from a few catalogs. Off topic sorry lets see the pics. Also why would you not internal wire your bars?

 

I like the look of wires hanging out, i dunno, i may wire it internally...

 

I had 14 " Nash Narrow base Grit hangers on my Sporty .I think most Nash bars do not have knurling.They slipped in the risers. Not bad but you had to retighten every now and again.Once I put some hockey tape between the bars and risers it very rarely happened again.Skate board tape ,sheet rock sanding screen would work just as well to prevent slippage.

 

I have Nash Pudgy Gimps 16", put them on right before cold weather, not many miles on them. Your right they are not knurled, call Nash and ask if they would knurle them for you. Most bars are made to order so expect a little wait. I have the Nash risers and hopefully the extra surface area will help keep them from slipping. No probes yet, but not enough seat time with them to give real opinion on slippage. Quality is great.