22mm is a standard plumbing size for cistern overflows, so you could bung a capped PVC pipe on it for a posh job? But cheaper still, find a bath plug of the right size and Araldite or seal it in (preferably from the inside). If all else fails, stick a patch of rubber sheet (eg an old inner tube) on it with sealant from the inside. Water pressure will help keep it watertight.