If your gun is a smoothbore then the cheapest route would be a saddle scope mount.
You are not apt to be shooting cloverleafs with a smoothbore and foster slugs anyway so a little bit of potential inaccuracy added by the saddle mount is not going to hurt anything.

Just make sure of a couple of things.
1) Your barrel nut is tightened down well.
2) Don't remove the barrel from the gun once you have sighted it in.

Any movement between the barrel and the receiver will cause your zero to drift.