A step wedge is such a low cost, high value photographic "instrument" that it is worth buying one rather than making one. I paid $3.75 for mine 35 years ago and though I wanted to buy two at the time money was hard to come by so I only bought one. I still have and use that strip regularly.
If you are in a "desert island" situation and must make do with tools at hand, you could make a continuous-wedge. Later, visit someone with a densitometer and circle spots on the negative that read in .15 intervals from 0.00 to 3.00 for a 21 step scale.
By making a continuous tone wedge and picking spots, you will have a better chance of finding spots on the negative that are the correct densities. If you tried to make steps, there would be many mis-steps even if you work to high standards.
Use Lith film and do an exposure under the enlarger of increments increasing in exposure time. You will have to do the math, but one way would be to expose for 10 seconds then start covering in steps and expose for 4 seconds, the 6 seconds then 8 seconds then 12 seconds. That will give you the series 10, 14, 20, 28, 40 seconds. You will have to use some trial and error. When you get one sheet of film that looks good, read off the densities just as you would an uncalibrated strip you bought.
I don't know. Seems like a lot of work for something that's actually pretty cheap to buy. I'm with Bill on this one. Even a calibrated one is reasonably priced. But the uncertified ones from Stoufer are just as accurate as the certified ones.
I agree with the stouffer supporters. I bought an uncalibrated 120 format 6x6 one a few years ago, and it was money well spent, and a most efficient organization to deal with.
I still use the 120 step wedge on 4x5. My denistometer can read the smallish stripes this size gives when contact printed onto 4x5 film
Just last week I made one of Ralph Lambrecht's reflection step wedges for field print tone pre-visuallization, and it took a bit of time to work out my setup.
To try and get it done as a neg, and then worry if I was off of the toe, or over the shoulder for linearity of the film response would just not be worth it when there is a most viable commercial source.