For a square groove, grind a hacksaw blade to fit the groove. Do not let the blade get hot while grinding. Grind all three edges with the wheel turning in the same direction against the blade, try to grind the edges square to the flat of the blade. Do not remove the 'curl' of metal left by the wheel, that is the cutting edge and the side that should be facing you as you pull the blade through the groove. Slit a dowel for a handle and use friction tape to hold it in. Pull the blade broadside through the groove with the handle angled slightly toward you.
Same process will work for round or vee grooves but much more care is required for both grinding and for scraping to get an even finish.