Rear cargo floor Y61

Hi all, finally got around to starting the marine ply fitting to the rear cargo area in preparation to installing the fridge slide. Now I've been through a few videos on the net and had a look at the consoles website as to where people are bolting down their drawers and floors. The captive nuts under the carpet seem very weak for the job. The ones at the rear are little 6mm bolts and the fronts are 8mm. There are the two outermost middle that are 5/16 (weird but that is the closest match I can find), and there are the two seat anchor bolts that are 10mm fine thread. Now how are people getting the correct bolt length and which bolt holes are they using? I'm levelling the floor so I plan on packing the gaps directly under the bolts. There will be a layer of dynamat on the floor as well. They certainly don't have correct length/width/thread bolts at either Bunnings or Repco so will have to shoot down to one of the industrial bolt suppliers this week coming. I also wonder about countersinking as the ply is 18mm and if I take out enough for a good sized washer and the head of the bolt, there isn't a lot of meat left for strength (I plan on using button head bolts if I can get he correct length). The slide I've ordered is a Ridge Kitchen V1, the fridge is a National Luna 72L, so the floor needs to have a good bit of strength/rigidity. What are you guys using and what bolt arrangement did you use?