Here's the problem with the trip odometer. The re-set knob is just a touch too short. It's about 1-15/16" long, and needs to be close to 2-1/4". To remedy this I cut the knob and lengthened it with a small screw. You probably don't need a screw to do it, since you'll never really be putting much pressure on it, you could probably just drill it, put a short piece of a nail or something in it and glue it back together, but I had the stuff lying around and it's what I used.
These are the part's I used. A small drill bit, a 4-40 tap, and screw. Then I cut the knob off and sanded the end where I was going to be drilling the hole in it.
Drilled the hole in the end about 3/8" deep. I used a drill, but you could also put the bit in an X-Acto knife. Drilling plastic is really pretty easy, just make sure it's centered the best you can.
This is what it looks like drilled.
Then I put the tap in a pair of Vice Grips and tapped the hole.
Then the screw will screw into it.
Drill and tap the other end, cut the head of the screw off and thread each end in.
Later, after I've adjusted it to the length I want, I'll put some glue in them so it won't thread out, but odds are it wouldn't move anyway. You can also see that I didn't get the bottom part exactly centered, it's not a big deal, since you can just thread it in or out a bit to get it where you want it.