You could buy
one of my computer controlled MOSFETs from the retailer section. They are about as fullfeatured as you will get.
They can enforce semi-only operation (fires semi-only no matter where the selector is) with no modifying of the gearbox, so you don't need to worry about your version 3 mechbox binding/locking out when physically on semi like it's prone to do. (That can be a pain if you removed the selector or pinned it into semi, only to have it bind up.)