You might want to check out one made by Sunrise Specialty that's almost 36 in. by 36 in. This is from their catalog "... #891 shower pan which measures 35 1/2" length, 35 1/2" width, 6" height. Cast iron construction and anti-slip bottom for safety." Sunrise has a reputation for making good quality cast iron clawfoot tubs and I imagine that extends to their shower pans as well. Vintage Tub and Bath sells the #891 along with accoutrements. Search on www.vintagetub.com.

If you have room for a 48 in. by 36 in. one, Kohler makes the Kathryn. Search for "Kathryn shower base" on www.us.kohler.com.

hth!

Here is a link that might be useful: #891 construction specs from Sunrise site

Thanks for the link. With that bowl shape, though, the standing room shrinks quite a bit compared to the more "square" acrylic ones I've seen. I'm also considering Swanstone, but cast iron would be nice. Can't squeeze another foot out of the adjacent room, though!