I'm crazy, old, cheap, and nothing better to do with my time. I'm also the consummate DIYer, so I build everything I can. I build my own frames, forks, and wheels. This provides many opportunities for tool building- frame jig, fork jig, rear triangle jig, truing stand, etc.
Here's my homebuilt ERD measurement device- hopefully others can get some inspiration from this.
The first image shows the measuring device mounted in the rim. It consists of two spokes fixed to a 6 inch machinists scale (metric scale on one side). The spokes are attached to the scale using fender strut bolts, so the spokes can be adjusted to accomodate different rim sizes.
Second image is closeup of scale and readings. Sorry about the crappy pics!
The scale is drilled for the fixing studs as close to the ends as possible.
The spokes are cut so each, with the nipples bottomed on the threads is 250mm.
This gives a total length of 500mm with the spokes fully inserted into rim and nipples bottomed out.
Read the measurement between the spoke ends on the scale and add to 500. That is your ERD.
As you can see- one spoke is fixed (left in pic) at 20mm on the scale. I couldn't fix it at "zero" due to the limitations in drilling the scale for the studs.
The other spoke (right) is "adjustable." Pull it taught in the rim and clamp the spoke to the scale with the stud. The measurement you seek is the difference between the two spoke ends.
In this case that is 48mm, plus the 500mm for an ERD of 548mm.
I did my guage this way because it was too cumbersome trying to hold the spokes tight in the rim and collecting the measurements using other methods.
It's a lot easier to use than it is to explain. It folds up for storage.
C'mon let's see some ideas from others. Maybe we need a thread primarily for home-made tools?