You're asking about installing a self-contained
system? The most common ones are portapotties, which are available from any marine catalog for a very reasonable price--far less than you could make anything. The toilet and tank are together and require no plumbing. The tanks can be carried off the boat to be emptied, or can be permanently installed and fitted for pumpout and/or dumping at sea. The most popular ones are made by Thetford and SeaLand.Read about them here: http://www.sealandtechnology.com/productpages.asp?pid=41and here: http://www.thetford.com/marine_series.cfmOr, for something a bit more elegant, see the Sealand 711-M28 Marine Traveler here:http://www.sealandtechnology.com/productpages.asp?pid=37 All are available in Europe...search the sites at the links I've given you for locations. Although with the current rate of exchange for the USD against the Euro, it might pay you to order from a US discount retailer.