How to attach thru-axle wheels to a truing stand designed for QR wheels?

There are several ways to attach a thru axle wheel into this truing stand.

One possibility is to buy a tool intended for converting a thru axle wheel to have an interface similar to quick release, to be used in truing stands (these adapters are not intended for use on the bike). Shimano makes two of these adapters: TL-FH12 and TL-HB16.

TL-FH12 costs around 30 euros, and converts a 12mm thru axle hub to have 10mm protrusions at the end. It also allows one to fit a quick release through the adapters, tightening the wheel securely to the truing stand. The main problem with this adapter is the cost: it only works for 12mm thru axle hubs and for many people not earning a large salary, may require one to work for several hours to earn enough money to pay for the adapter.

TL-HB16 is even more expensive: it costs around 40 euros, and works for only 15mm and 20mm front hubs. It also allows putting a quick release through the adapter to tighten the wheel securely to the truing stand. Similarly to TL-FH12, the main problem of this adapter is its cost: it's even more expensive than TL-FH12.

A cheap solution to emulate a 12mm axle having 10mm interface at end is to use a long 10mm Allen key. This Allen key has 10/cos(30*pi/180) = 11.55 mm distance from corner to corner, and fits into 10mm slots. This approach has a drawback that 11.55mm is bit smaller than 12mm so there's a bit of free play at the axle. If selecting an Allen key that is tight fit (one that's intended for undamaged Allen head bolts), the 10mm fit is a very tight fit to the 10mm slots, so not only gravity but also friction is holding the axle securely at the lowermost position. In practice, with this kind of cheap truing stand, the sloppy 11.55mm - 12mm fit doesn't create problems far above to the problems a cheap truing stand already has, so one can build wheels that are accurately enough trued for even rim brake bicycles (although those tend to not use thru axles). I have built four such wheels using this Allen key hack, and they are excellent, very durable, far better than machine-built wheels or wheels built by a wheelbuilder having unknown skill level.

A better solution can be made with some woodworking. Hardware stores sell at least 12mm and 15mm round wooden shafts (20mm round wooden shafts seem to be hard to find at least where I live). These wooden shafts can be cut to the desired length: bit over 100mm for front hubs, bit over 142mm for rear hubs. After sawing the desired length of the shaft, the shaft can be pushed through a used hub with black grease having wear particles in it. Most likely one observes that pushing the shaft through the hub requires a lot of force: that's because it's actually not 12mm all the way around. At some locations, it may be as sloppy fit as 11.95mm and in other locations it may be as much as 12.05mm. However, after successfully pushing the shaft through a used hub with black grease, the shaft has black grease marks on those locations that were a tight fit. A grinding machine can be used to remove slight amounts of material from the locations that were a tight fit. After successful grinding, the shaft can be pushed again through the hub axle: this time it should slide rather freely with little resistance. Then the ends of the 12mm or 15mm shaft can be ground with a grinding machine to fit the 10mm slot in the truing stand.

The Allen key approach has a disadvantage that it's a sloppy fit at the axle-to-hub interface and tends to scratch the Allen key surface making it very ugly. The Allen key can't be tightened with a quick release but usually friction is enough to hold it securely. Also Allen key only works for 12mm thru axles and not at all for 15mm thru axles or 20mm thru axles. The woodworking approach lacks the sloppy fit disadvantage and while it can't be tightened with a quick release, making it a tight fit both at the axle-to-hub interface (by grinding only the necessary amount of material away) and at the axle-to-slot interface (again, by grinding only the necessary amount of material away, checking the fit very often and stopping when the axle just barely fits into the slot) makes it good enough to not require any support from quick release.

The woodworking approach can be extended also to create a full custom truing stand from wood: one can drill 10mm, 12mm and 15mm holes to the wood to hold axles of various sizes (QR, 12mm thru, 15mm thru), have a base where the sides are screwed with an L bracket, one side being movable with screw holes (to hold the L bracket) for 100mm, 135mm, 142mm, 150mm, 197mm etc hub widths. Then the axles can again be made from 12mm/15mm wooden shafts. In this case, it's beneficial to "sharpen" one end of the wooden thru-axle with a grinding machine so that the sharp tip guides the wooden thru-axle from the hole to the hub, and from the hub to the hole at the other side, without needing to guess perfect alignment. The wooden thru-axles for the custom wooden truing stand don't need to be shaped with a grinding machine to fit a 10mm slot like for the cheap steel truing stand.

Adaptors for running TA wheels on QR frames and forks are readily available for cheap on websites such as AliExpress.