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I've never done a whole bird before. I use pieces, and slashing them is an important part of marinating them. I prefer wings and drums. Spatchcock will probably work fine as long as you allow the marinade paste to penetrate the skin. I used to make my own marinade but I've found the jars of pre-made store bought to be pretty good so out if convenience I use them. Marinade for 24 hrs for best result. I go indirect ~350 apple chunks for smoke btw.
If you want to make your marinade from scratch, here is a recipe that I loosely follow:
Just cook the Spatchcock like you normally would but be careful because the marinade has a bit of sugar in it. That will burn easily if it gets to hot. Let us know how it turns out!