What is it worth to you to avoid drilling the joist and getting the pan you want? You might get a quote from a place that come up on this search engine search: "custom shower pans" price
I am not saying that including the word price in the search will result in a price in the results, but it seemed to help a bit.
I am not saying that including the word price in the search will result in a price in the results, but it seemed to help a bit.