After a while using EtchASketch's whizwheel for targeting, I decided to try and find a 'nicer' looking, and perhaps more accurate, solution.
I found a website that gives you the templates to make your own slide rules, both linear and circular. You can find the .pdf files here:
http://www.sphere.bc.ca/test/build.html
Here is, from top to bottom, the following:
1. EtchASketch's 'Whiz Wheel'
2. A cardstock simple 4 scale slide rule
3. A Yokota-based Log-Log made out of the box from a Revell Type XXI model
A couple of observations:
A circular slide rule is probably the way to go if you are using cardboard. The one on the bottom has a pronounced curve to it (but it does work).
I am planning on getting some Lexan from the local Home Depot and making a few of the ones on the website. I'm still learning the theory behind them, so I'm not sure which of them you could use to replicate EtchASketch's design.
Using a stiff plastic would prevent the warping. If you are handy with wood, that would also work well.
Good Luck, and Happy Hunting!