Some more progress on the new design. In this design, I lathed the center of an original selector button completely out, so that no matter what position it is in, it cannot act against the disconnector. Then I drilled a small hole in the remaining flange, big enough to hold a brad nail.

Next, I inserted the selector into the Sten, put in a section of steel rod cut from the above brad nail, and then MIG welded into the selector so that the pin cannot be removed. Now the selector can never be changed for an original un-modified selector and no matter the position, it will not press against the disconnector. This, coupled with the narrow disconnector slot, should go a long way to ensuring the gun remains a semi-auto, but the selector button still moves for nostalgic reasons