The rusty cast iron drain cleanout in my basement is cracked and I can't seem to find a matching piece at Lowes/Home Depot. The cover is about 5" wide. What would be the best thing I can use to seal this off?
I think I'd use a 1/4" aluminum or 1/8" galvanized steel plate with a layer of rubber or neoprene underneath. This way it's easily reusable.
Just drill holes for the studs and use new galvanized nuts and suitable washers.
Get an Abs or PVC (better) Cap of a pipe bigger than your 5 inches to drill two holes through and use high-quality silicone to create a seal on the old metal. Clean the old metal surface with a wire brush (Dremel). Get new washers and nuts and you are in business.