You don't really want a flexible holding tank!
For a whole bunch of reasons, not the least of which is their nasty tendency to blow out fittings at the first sign of a clog...and you'll find more good reasons in the articles in the reference library in the Head Mistress forum. We'll be glad to help you with top quality rigid poly tank that fits the space...we offer nearly 400 different shapes and sizes, including over 100 that are non-rectangular. E-mail me at peghall@bellsouth.net and I'll send you a catalog. We're distributors, not mfrs...so I don't have any axe to grind by recommending one material over another..If flexible tanks were suitable for sewage holding, we'd sell 'em.