Fersure do the dimple thang with a sharp pointed center punch...Pound lots of dimples

The center punch makes a deeply pointed hole in the bar that also raises metal around the edge of the holes. It's the raised metal that allows the riser clamps to bite.

It's an old trick I learned when building chevy small blocks to keep the intake front and rear gaskets in place.

Forget any thoughts of grinding or using dremel to cut groves. Cutting grooves does not raise metal, it simply cuts. You have to raise many sharp metal edges that increase the bar diameter if you expect the softer riser clamps to bite.

As a lesson learned, never buy any bars that are not deeply knurled. Those that are not knurled are cheap knockoffs than can slip and kill you.