Can keels be installed on cut barrels to keep the opening between barrels and ribs from becoming a dirt collector? If so is there a source for the keels or are they generally made in shop? Is there a better solution altogether (other than not cutting the barrels in the first place)?

They would just be made out of whatever steel you have laying around.

They can be shop made. I make them from mild steel, solder in place and strike off. They area can be filled with solder as well. You need to put a small amount of steel wool in the recess first to catch the solder. I have never seen keels for sale.

I have made dozens just filed up from round bar , welding rod is OK. A triangular shape of some form will do for most jobs and solder will fill the rest .