I found my stock yz250 triple clamp and its hollow too. See what you mean.
Shoot you could dremel out the stock.......(wrong....just measured with calipers my bolts are 1.040" apart. yours might be further apart?)
I thought Mac would be a rare exception but seems like 3 people right away like the bars. Looked on RockyMountain the flexx bars have 5 stars if that means anything. That would be one option is dremel the stock triple clamps. You get the anti vibe with the flexx bars.
If it was me i would grind off the mount on the triple clamp. Then find someone who welds aluminum to trap a nut underneath. Or if possible weld and fill the bottom of the triple clamp with enough material that you could drill it out to accept a normal rubber mounted 1 1/8th handlebar clamp. Run the flex bars or any 1 1/8th you choose.
Ive had Kevlar grips that lasted several years cause i barely ride. Next set will be super soft half waffle. That should help with hand fatigue. I get to the point out in glamis that i stop using the clutch for safety. Because trying to open my hand once its gone numb is scary. Cant really tell if youre squeezing hard or barely at all. It sure feels like barely at all. Which i think makes you squeeze harder making the problem worse.