This might work for someone.  Materials: 1" x 1" aluminum angle 1" wide, 2 quality plastic zips.  1) drill and file two rectangular holes for the zips. 2) carefully curve one side of the aluminum to match the shape of the twist throttle. 3) Attach zips as shown, position aluminum tab where it feels right before final tightening of the zips. 4) run some CA glue between the zips and aluminum to prevent the new throttle from twisting sideways.  There is NO gluing to the original throttle.   I've enjoyed this thumb throttle mod for 400 miles now without issue!

My experience:
For my riding on the Phoenix East Valley pathways and roads, the RadCity 5Plus standard 2.0" smooth tread tires, are inadequate.  The 2.30" knobbies I installed (58-584) made a great  improvement in handling and tracking.  Per Rad Customer Service, 2.30" is the largest tire that can be mounted on the rims.  If you're considering knobby tires, beware of tires that have large knobs on the outer edges (like eRubans).  These cornering knobs interfere with the fenders and fender support brackets.  With modifications though, such tires can be made to work.  Hope someone finds this knowledge useful.  ((No, I didn't want a fat-tire bike))