The Holmegaard one (proper name is klukflaske in Danish, and were marketed under the 'Cluck-Cluck' moniker in English) would have originally had a clear, uncut, solid ball-shaped stopper. You can usually find stoppers along the correct lines at antique fairs , and sometimes on eBay in joblots of other stoppers.
Getting them to fit is a pain, but you could always send off a suitable and slightly over-sized replacement to a glass restorer and have them 'shave' some glass off to get it to fit for a relatively small amount. Or, if the decanters are terribly special ones, you could likely get new ones blown for you for around the £30-£40 mark.