Cut the continuous rotation servo arms down to about 4mm each.

Trim long... you can always remove more if it's not enough.

If the fit of the servo horn is too tight, warm the wheel with a torch and press it into place.

Until the back of the servo horn is flush with the back of the wheel.

Use the thick wheel hub and long plastic screws.

Tighten the screws evenly - keeping the wheel perpendicular to the servo.

It may be easier to guage depth from the back side of the wheel.