I used to have a commercial one that clamped onto the frame. The push bike shops sell them, theyre under a tenner.
If you want to make one up, two arms longer than the radius of the max size wheel you want to true, metal slots at one end to fit wheel spindle and some cross tubes, one arm with a slot in, and bolt a block with a cross tapped hole into the slot (the slot is to accomodate different rim positions), then into the cross tapped hole insert bolt with point on the end to form the indicator. In fact, if you have a old frame, saw off the frame forwards of the saddle tube, remove the bottom bracket and weld a series of nuts to the frame to correspond with different rim sizes, you have a instant ready made jig.
A friend used to build motorcycle wheels up for people commercially, and that is what his jig consisted of..
I made one 20 yrs ago using the Park Shop model as my basis, This is a self centering model and makes the job easier. These are quite expensive to purchase and so made an easy choice to build. It is mainly a c-channel base with the fixed and moving uprights cut from two sizes of box stock. The hardest part of construction was the adjustment rod which has both right and left threads on it, I made it from a turnbuckle. Fell free to e-mail if you'd like more info.
Anyone got any photos of these things? I'm struggling to get my head around what a truing jig actually does. I always used the cycle forks for mine. (theres an add photos button below the box you write the post just in case anyone isn't familiar with the forum, though I guess 20 years is a long time to keep hold of such a device).
Go to parktool.com, Click on products-wheel & tire tools-wheel tools- scroll down to truing stands. The one I made was like the TS-2. I noticed a rebuild kit listed, that would make job much easier. Still quite a fabrication but the end result was worth it for me. Yes I still have it and use it often. Ive built over 5000 wheels on it.
I made mine with a pair of old forks stuck in a vice and a nut and bolt welded to the forks were the rim lines up.
The nut is welded allowing the bolt to wind in and out to sus out the wobble.
The dti will be overkill, the pointer will move so fast even on what looks to be a well trued wheel you'd be lucky to keep up with it.
Unless you found one that read in graduations of 0.1mm or something...
Seen this article which might be of use.
Particular of fascination to me is the fact they use a ring roller with special wheels to make the rim from a flat sheet of 6061, then weld the ends of the ring together... http://www.motorcyclecruiser.com/tech/wire_wheel/index.html