I see there is someone on Ebay selling converted 1000 litre water containers (the sort used by farmers) for use as septic tanks. These seem a bit small to me but I wondered if this could be converted:
http://www.njuskalo.hr/grijanje-hladenje-ostalo/spremnik-loz-ulje-2000l-oglas-8402714
http://www.njuskalo.hr/grijanje-hladenje-ostalo/spremnik-loz-ulje-2000l-oglas-8402714