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Your focuser will likely be longer than 2cm so your tube length will be shorter than 30cm but it depends on the type of focuser you had in mind. I made a 50mm finder from and old binocular lens and some plumbing pipe. I used some threaded plumbing reducers to step down from the 50mm pipe to a diameter suitable for holding the eyepiece which was the eye lens from the binocular. (The binocular prism was damaged, that's why I had the lenses spare). The focussing is done by turning the helical thread on the reducer. Even with this setup the focuser is approx 5cm.
Be prepared for some cutting as it's more likely than not. Note if you want a diagonal your pipe is shorter again.
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