Patterns for shell jackets are not difficult to come by on the internet. Some are more authentic than others, but that doesn't appear to be a priority for the purposes described here.

Advantages of sewing one's own shell jacket are personal selection of fabric, accoutrements, and DIY. Same applies to sewing one's own kepi or bummer head covers.

They are not particularly difficult to sew - much less complicated than a modern sports coat.

Also - I recall seeing illustrations in visitor centers at battlefields in Scotland that depicted government Highland soldiers wearing cartridge boxes where sporrans were normally situated.

Slainte yall,
steve