Thats what i just did the other day....took a flat piece of steel about 1/16" thick and cut a 1.5" piece out to use to hold the adjuster nut while tightening the lock nut with a torque wrench and a 1.5" socket i found at pep boys. (found the 3/8" to 1/2" drive adapter at autozone)
By the way, Dave makes a very good point about this procedure i think everyone who's going to do it should be aware of. The triumph manual says to lift the front wheel off the ground to do it. WRONG! It is definitely a mistake in their logic and i can attest to this. The way the procedure is done is to tighten the adjuster nut to 40Nm then back it off till loose, then tighten to 5 or 6 Nm. (i think it's 6) That 40 Nm seats the bearing. Then when you loosen it and the seated bearing stays where it is you then apply 5 or 6 Nm to keep any slack out but allow free movement w/o binding. Heres where thier logic is wrong.....If you have the wheel in the air, after you seat the bearing with 40 Nm of torque and then loosen it, the weight of the front end will pull it down a bit and unseat the bearing. So when you retighten it there will be too much slack and you are back where you started with a loose headstock. I did it per the manual so i know because it was still loose when i rode the bike. And when i redid it the proper way with the wheel on the ground, as soon as i undid the locknut i saw the adjuster nut not only was no longer torqued to 5 Nm, it was totally loose to where i could turn it by hand? Why? Because like i said, with the wheel in the air when you loosen the nut after seating the bearing, the front falls down creating slack that is exactly what you were trying to get rid of ! The nut will SEEM tight because the weight of the front end is pulling it down against the nut, but as soon as you let the bike down to the ground it pushed the head rod and all back up creating the same slack you had tried to get rid of. keeping the wheel on the ground as you do this eliminates that from happening.
Why triumph detailed it like that in the manual is beyond me. But when talking to Dave he said he had owned models with the same procedure and same setup that said to leave it on the ground. I believe triumph screwed up somehow. Someone along the chain of people on up to the ones who actually chose the words in the manual was wrong and that made it into the manual wrongly.
By the way, if like me you go out to buy a 1.5" socket, heres something to consider....i found i could NOT use it to torque the adjuster not. All i could use if for was to tighten the locknut. The reason is that it wasn't a deep enough socket and the threaded headstock tube was too long so the socket wouldn't reach the bottom nut, only the top one. (and yes, i had taken the top nut off when i tried) So before you buy one consider that and look for a deeper one, which may be all but impossible to find. I intend to try and ream mine out a bit with a dremel and some grinding tips, but it will be a long hard process with a lot of broken tips, and even then i may well not be able to do it.