thats from the artbook right? thats very awesome... that big daddy though actualy was never used in any of the games, it was just a model used before they decided to put in the rumbler in the second game. but i can help you out a bit. i cant give you any pictures but i do know for a fact that from loking at him about 100 times in the artbook, his back most likely has the tanks with the guns connected on to it (and it looks like its got some tubing connecting to his tank to supply oxygen to him inside the suit. honestly, just do pretty much what you think his back will look like because the back was never actualy decided for the suit so there isnt alot of pictures or description of him. the picture in the artbook is pretty much all ya got! anyway, i am also building a suit for the release of infinite, but im gonna be subject delta from the second game. ive got a thread on here about it, you should check it out because its got alot of tips that might actualy help you on building a big daddy costume. good luck mate!
edit: oh wait you wanted someone to come up with a back for you? oh man, i could do that for you easy! of course it wouldnt be the best drawing in the would but i could easily draw it out for you.