We've built a much dirtier/quicker gauge for use on a variety of cars here in the shop. We use only a piece of electrical conduit to which we've welded a pair of triangles made from welding wire, plus a pair of wires that extend upward from them and bend at right angles at the top. We proceed in a slightly different way:
We jack each front wheel up in turn and spin them while holding a piece of chalk (on a jackstand or such so it can't wander) right on the center (not critical) of the tread. This puts an accurate measuring reference on each tire. Then we shove the checker under the car and roll it up until the bent end of the wire contacts the tire. We hold one wire on the line and bend the other until it does the same. Then we roll the rig back down, whip it out from under the car, and apply it to the front of the tire. From there, the procedure is the same. The reason we do it this way is that we don't trust the outer rim to run as true as the adjustment we are making.
The rig decartwr has made looks quite sturdy and I'm sure it will repeat any measurement put into it. I'd advise, however, that you check your wheels for runout and find a position for measuring wherein the "average" of the runout on each side is what you are measuring. On the other hand, if you have perfectly true MGTC wheels, you have performed a miracle and don't need to do this!!!
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon