why dont you think more modern instead of a bath why not give them a shower i give mine a shower in the loft i just put the sprinkler on the hose pipe . my loft is ten foot off the ground and grill floor and drains below but iam sure you could make a cubicle with pump and filter i made one for my dad years ago i used bottom off plastic 45 gallon drum a piece of clear polythene stapled to inside of bath then i used three washer motors from scrap cars and car battery and three pieces of rigid wire to hold polythene up so its like a tent use windscreen washer pipe from scrap car and its colapsable . or you could use fish tank motor any way just a thought for you to consider lol