If the side vents are just convection, may not be good to block them off, it might get too hot. If you have a way to harness the heat or tie it into the top vent/duct, that would work.
On the burn plate, I don;t think it really matters, every stove is slightly different, but all work on the same principle. Being more horizontal, it helps keep the coal from sliding off when you trying to light it. But, I don't think makes any noticiable difference. it might just the way the stove is designed. My LL has a pretty good angle on the burn plate vs. the Keystoker being almost horizontal.