On the suggestion of Cajun Cook and Frank Robb I bought these some years ago and they work fine. Current price is $3.66 each.
#5944A43 Tight Clearance Slotted Insert Bit, 1/4″ Hex Shank, 3/16″ Tip Width, 1-1/2″ Overall Length, 1-1/8″ Shaft Length
https://www.mcmaster.com/5944A43/
