Your handle clamp base can crack if the 4 bolts are not tightened evenly. The plastic base can be quickly replaced with metal straps:
1) Cut a pair of 3-hole long staps from galvanized strapping
2) Round the edges of the straps
3) Attach loosely one end of each strap to the upper round handle with original bolts and nuts. You can add a washer on strap side if you like. Slip handle over drive tube and bend the straps around the tube to line up with holes in handle.
4) Put bolts through handle and strap and tighten all 4 nuts snugly and evenly to pull strap against drive tube. Just tighten nuts enough to stop movement of handle. No need to over-tighten.