There is another instructable that uses a dial gauge and a similar inexpensive stand. That instructable actually inspired me to consider modifying my own! But do you really need a dial gauge? From my perspective the answer is "no". Just buy a cheap set of "feeler gauges" that include 0.2mm and 0.5mm sizes. With the precision of the screws we've installed, you can insert a feeler gauge between the indicator and the rim. Then spin the rim. If it doesn't touch the entire way around, your wheel is true to that maximum error.
If you want it to look mildly prettier, you could conceivably file/dremel the edges off of the "second" nut and insert it into the plastic, then press the brass insert in over it.