I have this same shell and also an ID Pullover in the eVent and my old GT parkas are now never worn. I have a lot of ID gear and nobody makes better stuff, IMO.
If, Evan Jones, the owner will build you a custom-sized item, (he has told me that "sometimes we will and sometimes we won't"), in one of the many phone calls I have had with him, this is the best option.
Marie of "Wildthings" has also and very graciously done custom-sized work for me and I prefer the products of these two firms to anything else I have seen on the market, so far.
I spoke with Evan last week concerning the new eVent shelter he is introducing and I am very excited about this, but, he said that he expects it to be offered in the spring of 2010.....worth the wait, I think.