We found a 4-inch uncapped cast iron clean-out behind wallpanelling in our basement (surprise!) - it is almost flush with the cement floor - and within a couple inches from the foundation wall leaving no room to install a screw plug. We don't want to leave the clean-out open when we repair the wall nor do we want to work with oakum/lead needed for cast iron fittings. Any suggestions on how to cap it? Can you use a low-expansion foam caulking/sealant?