Williams has an inexpensive receiver sight model for the Krag that mounts  on the right side  of the receiver. It is drill and tap but only if you want it to be. I had one on my sporter (see SN 456713 thread) I attached with JB Weld along for with a regular front sight base for 12 yrs. 

There are also old EL Rice peep sights out there that mount on the extractor and are less than other options mentioned (see same sporter thread for picture of a Rice sight).

May not be ok in regulated shoots but for casual shooting you will see better than with the issued sight options.