A jig can be made very easy and doesn't have to be a tool room example, although not for a Barr & Stroud the pic should give an idea, as Peter said using balsam you can redo till the cows come home.......
A very simple jig below, Heath Robinson yes but does the job....
A link below is a repair to Binocular Prismatic No5 may be of interest to some, includes the home made Jig above.
https://www.milsurps.com/showthread.php?t=53576