The Springfield M1903 has hit the market, but only in the army version - Weaver scope. Here is an attempt on the Unertl scope as used by the USMC. Those should not be confused, the rifles themselves also differs slightly.
Two versions vare included: One is a shell around a cheap(er) Umarex scope, and if 3D-printed in PETG it should be sturdy enough. The other is straight up for reference.
There are several options: If you design the outer tube in two halves, setting aside room for the active part of the original post adjuster, you should print the Unertl posts as solids and make everything as rugged as you can.
If you decide to use the original Unertl system for adjustments, and fix the post in the inner real scope and do some sanding, threads for the Unertl parts are there.
Any pellet gun or air soft scope slim enough will do, so you will need to tweak and adjust for fit the 'scope' as you would have it..! Most of the work has been done, still some creative fun is left for you. Coded in SKP for absolute beginners. All parts tested as a solids.
Happy tweaking..! Even a repro Unertl scope is quite expensive, so this could be a good alternative for a dedicated airsofter og reenactor.