I slotted two lengths of angle iron from an old bed frame and bolted them to a scrap 2x6. I've built and trued many a wheel this way. The angle iron flexes enough it works on everything from 126 mm QR's to 142 mm through axles.
To visually gauge run out I hold a metal rod in place with a magnet and back light it with an Ikea clip on LED lamp. As the wheel spins the distance of the shadow from the rod clearly shows any run out without anything scraping on the rim.
The Ikea LED lamps make excellent shop lamps. I use them in lieu of drop lights.