I finally got to work this weekend building mine. I ordered the IMUSA tamale steamer from Target through Amazon since my local Walmart didn't have any in stock. It arrived in a few days, but alas, it did not fit as others have commented. It has a 14-1/8" width (at the base), 11-5/8" height (w/o lid), and is made in Columbia. I can return it to Amazon, but I may just keep it around so I can experiment with a modification that should make it work.
There are a number of Mexican groceries not too far from where I live, so I decided to look there. I found another 32-qt steamer at the second place I visited, and got amused looks as I whipped out my tape measure to verify the width. It was 13-3/4" wide and 12-1/2" tall, just about perfect. After measuring my smokey joe silver, I was hoping for something closer to 13-7/8", but this one turned out just fine. It cost $29.99 + tax, the brand name is Alpine Cuisine, and is made in China.
At the same store, I noticed several even larger size steamers, something in the 40 qt and 50 qt range I believe. I later wondered if these would fit any of the larger Weber kettles, like the SJP...
The hardest part of modifying the steamer was getting the darn label off! I didn't want to mar the surface too much so I used some leftover wallpaper remover which allowed the paper label to come off much easier than without it, but the adhesive remained. So I used some paper towels and lamp oil (kerosene) to remove the glue.
A variable speed jig saw with a fine tooth metal cutting blade made cutting out the bottom easy, accurate, and neat. I still have a few things to do before I can test it out, like figuring out where to set the grates, but I should be up and smoking soon.
Thanks everyone for contributing to this interesting and helpful thread. Cheers!