Get both dimensions off of the ID website, I'm sure they are there, overall diameter and cutout diameter. Then tell the guy that you need a piece of mdf with a circle cut out to the cutout diameter then glued on top of that one cut out to the overall diameter plus 1/2" and inch or so, this will give you 1/4" larger all around if you plan on carpeting the box and such it won't be as tight since the carpet will take up about 1/8th of an inch. Then not sure exactly how your design is going to be but about 2" larger on the outside should work.
Best bet is probably find a picture somewhere of a similar speaker ring and explain to him what it is going to do, maybe even a speaker ring with a sub loaded into it, let him know the cutout and overall diameter of the sub and he should be able to handle the rest.