(Phase II in center, Phase I on right - both fitted with JoeyB inserts)
Ditto JoeyB's plastic inserts. There are none (other than a gen) better, but they are designed to fit a gen or gen-dimensioned bezel, so, unless it is made to gen spec, you would need to measure your rep/aftermarket bezel to be sure the insert fits.
As others have said, '42 builds are complicated & expensive. The options really come down to spending thousands on a mixture of gen & gen-spec aftermarket parts, or heavily modifying a rep Sub or GMT case (these are generally made to fit ETA or ETA-clone movements, instead of gen Rolex movements) to mimic the '42. Several years ago, for my Phase 1 '42, I used a cheap Silix Sub as the basis for the build (I do not recall the exact rep model, but I believe it has been unavailable for several years - if you can locate the model in 1 of my Phase I construction threads, Silix may be able to locate a similar rep for you). It was selected because the basic case was close to the '42 in size/dimension, but the crystal & bezel mounting were different, which required a great deal of modification & input from other advanced members (you can search out the several threads on the subject for details).
Whichever way you decide to go, be prepared for alot of headaches & budget excesses.