Cut the mortise in the butt stock under the trap for the spanner wrench. At its deepest the mortise is 1/4 x 3/4 x 3 1/2" deep. And may I add into walnut end grain. below are a couple pics (note: the screw will be machined to be the pin portion of the wrench and only stick out about 1/8" when done)
I need to consider how and what to line with under the trap door and down into the mortise
[Linked Image from i.imgur.com] [Linked Image from i.imgur.com]