Great pictures and installation. I can understand what you are saying about trial and error, not many books to tell you how to do a project like this and getting the wiring run so it doesn't get in the way of anything is tough sometimes. I need to caution you about one thing though. As euro-bred said, you really need to seal that box and vent it to the exterior of the car, down through the floorboard will be fine. We do make ventable batteries, but the one you used, a group 35 (as best as I can make out in the pictures) isn't one that has venting provisions. You are correct, it is fine to mount it on it's side, but, any battery that is overcharged has provisions to vent itself and our batteries are no different. Without being able to vent, they would explode and that isn't a good option either. However, those gasses are toxic and can harm you and your passengers in the event that the battery did get overcharged. Please look at maybe a new cover for your box, some rubber gaskets or something to seal what you have there and then make some provision to vent it outside. Depending on the area you are working with, you may be better off to google ventable battery boxes and buy something that will be plug and play.
Thanks for posting the install though, very informative.
eCare Manager, OPTIMA Batteries, Inc.