I cut two pieces of angle iron from an old bedframe, notched the ends and bolted them to a scrap 2x6. The angle iron flexes enough it holds anything from my 126 mm QR rear wheel to the 142 mm through axle wheels.
The trick is a flexible LED light illuminating a magnetic pointer positioned near the rim. Need the light hitting the pointer from the side and projecting onto the rim. If the length of the shadow stays the same then the wheel is true.
I do have an expensive spoke tension meter to get the tension right. But once tensioned I pluck the spokes and tune them by ear to get them all uniform.
Won't win many wienie measuring contests like we have here but when used right does a damn fine job truing wheels.