There's tons of used chainsaw mills available. Post a wanted ad. Guys buy them and find out how difficult it is to make lumber with them and they hang up in the garage/barn/shed until they read your ad seeking one.
I have a Granberg Alaskan. It has a 24" capacity. Have I used it? yes. Often? no. Their place in my opinion is making specialty items like slabs for mantles, big beams, cribbing planks, etc. It's a very slow, tedious and fairly expensive way to slab wood. A band mill is way mo better but expensive. We had a GTG here several years ago and made a cant for some chainsaw racing. It works well for that too. Keep in mind you'll need a bar at least 4" longer than the wood width you're going to mill to allow for mounting.
It would be easy to make one if you had one there to copy, there's not much to them.