I recommend a strip or two of the duct tape in each axis around the case, but that may be overkill. Once taped, you can make a "floor" of cases in a raft under cargo areas. Out of sight & sun and low center of gravity for the boat. If they are down there not individually secured (which is fine), you need to rig the gear that piles above them extra bomber. Prolly a good idea anyway.
A drag bag works good on the Grand with regard to temperature (cold water!), but if the drag bag is too large, it will mess up your all-too-critical conveyor belt line and suck you into the giant 5 minute eddies. Couple of those and you'll be pulling that sucker up on deck. A small drag bag with capacity of 12 beers or less won't bother you so bad.
Separate smaller beer cooler is a must - do NOT go foraging into long-term food coolers for beer every half hour.