If you cut the drum 1/2" below the lid all the way around you could have a thin piece welded all the way around on the inside. That way the part you cut off will fit over the piece welded on the inside. May be a tight fit but a grinder and some elbow grease should fix that.
I was lucky enough to get a new open head 30 gal for $11 haven't even came close to starting it yet.
Good luck keep us posted.
UDS (not so U) UDD, 22.5 OTS, Coming soon: mini UDS & double barreled RFS