As @Alan writes, you want to put some backing. If you are planning on adding several cleats to the deck in a given area, why not put a piece or two of 1/4 plywood in place, and then throughbolt the cleats with decent sized washers. I think 1/4" is probably enough for anything other than perhaps vang and halyard, if in doubt, use two, stacked. 1/4 will bend easily enough to accommodate the curve of the deck (and if you need more strength, stacking them you can bend each one better than a single 1/2").
Now, I would make sure the wood is sealed using neat epoxy, and the same can be used to glue it in place to the underside of the cuddy. Nothing fancy needed for gluing it because the first set of bolts will then hold it in place. Do give it a coat of some spray paint to keep out whatever UV makes it inside your cuddy and preserve the epoxy. Finally, need to remember to seal any holes you drill.
That said, I've only mounted cleats on the aft edge of my cuddy (DS1) and they've not required any supports beyond washers in that location (see image). Other areas of the deck do have wooden backing for added hardware.