Your other option if you cannot find a proper size preformed pan is to do a mud base tiled pan. Yes, it's a helluva lotta work, and I wouldn't advise someone to try it who hasn't previously worked with tile, but that is one way to achieve custom sizes. Some membranes are available to take the place of a traditional lead or copper under pan, and there are also some plastic kits to assist with forming the slope of the mud bed evenly on all sides towards the drain receptor.
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There ain't nothin' more powerful than the odor of mendacity -Big Daddy
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