Frank sells plans for just such a trailer, might be worth a look.
I would find a local trailer dealership that stocks a few different brands and go have a look. Take your tape measure and a notebook and I'm sure you can get everything you need. They are used to people measuring their trailers especially when used for a speciality purpose like a custom built smoker. In the end, you don't have to buy it but no reason you can't look and snap a few pictures.