Hi there
Most finderscopes have a fixed focus so you can’t really adjust them to incorporate extra length to the image train (back focus). When you add a diagonal to the light path you are going past the focal point so you won’t be able to focus. As an alternative you can get a finderscope with a diagonal that’s built into the scope. Like this:
https://www.ozscopes.com.au/skywatch...der-scope.html
Hope that helps.