Hi delirous
This is one joint you don't need a jig for ,you can use the router table for this one but I don't recommend doing the joint with the router.
The snapshot looks like the stock is over 3/4" thick maybe 1 1/2" x 2 1/4" that said,I would use the radio arm saw or the table saw for this one.
This is why ,,,, the small part left in place on the end of the stock has a good chance of snaping off and must take real care with this part.
It must be sq. and true so when you put the lap joint as one it will not push on the end part b/4 you have the glue set in place.
The radio arm will let you cut a 1/64" or less and you will be in control unlike the table saw or the router table.
All it needs to is nick that out side part and it's gone on the router table or the table saw.
Now for the other part of the lap joint, that one no big deal with the router table just a pass or two on the table and it's done.
One pass and then move the fence back a time or two and it's done.
And the radio arm saw will do this part also.
If you don't have a RAS or a router table and all you have is a plunge router you can make a templet using 1/4" or 1/2" MDF or plywood and use the jig saw to cut a sq.hole out and use a brass or steel guide in the router base.
If this is the case I would use a SHARP hand saw and pre cut the stock by hand so you don't touch the out side part with the router bit.
It may take you a time or two to make the templet it must be right on.
It will snap off if you hit it with router bit...
Good Luck
Bj