Some mountain levers come with threaded 10 mm barrel adjusters. Interestingly, these levers also seem to be the type that gets frequently run into the ground, trees or brick walls. Unfortunately, we don't stock 10 mm barrel adjusters, as they are relatively rare.
Yesterday I had a repair that needed a new barrel adjuster, but it wasn't budgeted for or ordered on the tag. The simple fix is to add a large washer to a lipped ferrule, so that the housing will be held in place and not slip down into the lever. Of course, this removes the functionality that a barrel adjuster would give, which is unacceptable in a customer application.
The kludgey fix is to slap a washer on an undersized adjusting barrel so that it can't fall through, but then the post of the adjuster can wiggle around, which is very poor form. Adding a sleeve for the post is a good idea, but because the levers are slotted you need a slotted sleeve.
My solution is to use a barrel adjuster with an outer diameter of 10 mm with helical knurling. I used a drill and a vice to drill it out to an internal diameter of 6 mm throughout its length, which also removes the threaded post. This makes a perfect sleeve for a 6 mm standard barrel adjuster, and threads in due to the knurling. Putting a washer behind the locknut transfers the force to the lever housing, so it is just as strong as the original, and won't pull through.
The cost in parts and time is trivial, and the end result has all the functionality and strength (or more) of the original.