Ahoy hoy,
Get the 15 pounder. Anticipate the future. Save yourself alot of headaches. The way it is constructed at the factory is much better then any 'addition' you will try to stick on later. I had considered doing the same thing, till I saw one. The 15 will have the extension rivitted on, and the edges are lined with padding so as not to slash yourself. And it is full packed when you go 15 pounds thats for sure. And no you dont want to have to do 2 half batches in the 7 pounder. The first time you realize the steps it takes even with an electric drill, the hassle of dumping, refilling, cleaning up the unavoidable mess that will occur when you start dumping and refilling, You will realize that its better to do it once, and all at one time. Ive run 4 or 5 hundred pounds of grain through mine, and every time I do, I smile at the thought of all the hassles I saved myself from by spending the extra jack, and getting the 15 pound model. You will not regret it.
Oh, and just to clarify, I hate spending money on things I can build myself, but this is one instance where its better to let the guy whos building the product to add the accessory . The way its made, anything but a tin hopper is going to throw its balance off, HAS to be physically attached to the lower hopper, because, if it isnt, you will find out just what a mess pounds of spilt grain can make.
I give you this advice from my own experiences. But I say again, this is one instance where its not worth trying to save a few bucks because your going to "make your own" . Opinions will vary. But you really DONT want to make your own hopper.
Whatever way you go though, the barley crusher is an outstanding product. Its worked flawlessly for me over the last year. Very well made.
And good luck in your endeavors!