TomD
Or you could knock one of these out for a few bucks, the blade is from a 1/2 x 1/2 tool bit, they run around 4 bucks. Mine was pretty short, but you can get longer ones.

www.ebay.com/itm/HHIP-2000-0083-1-2-x-6-Inch-HSS-Extra-Long-Square-Tool-Bit/282241726895?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D38530%26meid%3D20b337e37d884795b8d85c031cb97085%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D351798109463

Since there isn't any mortising you can make one up in maybe half an hour.