A timer? What's wrong with your watch?
For many distances you can use parts of your hand/fingers to measure your way across the chart. My thumb from tip to first joint when bent ( & most other people's) is ~20nm on a 1:1 000 000 scale chart, 10nm on a 1/2 mil, 5nm on a 1/4. Makes a very quick & convenient measuring device.
SC, to address your comments to me, I spent 10 years teaching people to fly, at all levels from ab-initio to instructor to multi engine/multi crew IFR including theory. My last instructing position was as the CFI of a PPL, CPL, night, IR, instructor, multi & aerobatic school with some examiner privileges.
I think you've missed the point re my comments about accuracy: I stated that in my experience the CRs had a tight grouping that clustered around the CRPs median. That makes it MORE desirable to use in the exam, not less. The user is more likely to get an answer that agrees with the examiner instead of being one of the unfortunate outliers. I've used & taught both slide & CR type whiz wheels. My experience is that CRs are a more effective tool.