Have already looked up van Maele as well as van Malle, found nothing. But I will do further research.
For making a chamber cast Cerrosafe (TM) is probably best, it is marketed in the USA and imported at least to Germany. Or, you may try to find a source in the UK for "Wood's metal", that is the scientific name for the same stuff and it is used for safety seals on gas bottles fi. It melts at about 80degrees Celsius, below boiling water. Stuff a wad of tissue or so into the barrel and pour molten Wood's metal into the chamber and an inch or so of rifling. Push it back out with a dovel or cleaning rod soon after solidifiing, measure after an hour or so.
Or you may use the classic material, molten sulphur. Here you have to lightly oil chamber and bore. Sulphur casts are very frangible and may catch fire while melting. That stinks, so do it outdoors.