Contact Huntington Die Specialties they are the Custom arm of RCBS.
1-866 RELOADS But it will not be inexpensive.
Lee Precision will also do custom work so if you wanted a custom collet neck sizer
for .358 WCF they will do it.
If you find an off the shelf solution you are better off, Wonder if a 35 Remington die would work or just a carbide .357 mag /.38 spl sizing die? I can usually adjust my sizer to just do part of the neck by backing it off - have you tried that?