DocCasualty:  Original Krag sight blades are generally .050" thick.  The slot in the sight base gives a snug fit to the blade (so there is no wobble when fully installed).  However, you should be able to easily start the blade into the slot and easily able to tap the blade into finished position with a light plastic mallet.  I would recommend you lightly sand  the inside of the slot of your base so the blade can be easily started into position.