I ended up going with a t-handle truck cap style lock. I build a simple latch plate for it on the inside of the galley. It isn’t as low profile as I would have liked but I do like that all the hardware is internal and I don’t have to worry about the hasp or lock banging on the camper.
I originally purchased a cam lock like the one’s Bill used but I was a little concerned that the 1/16th or so rim wouldn’t have enough grab to hold and not pull through the hatch. I’m sure you could make or find a washer to help spread the load a bit if you wanted to go that way.
Do you happen to remember the hardware you used for this? I’d love to order the same handle/lock but I’m having some trouble finding it. Also, does the hatch feel secure enough with just one latch point?
I don’t remember where I got my specific one but it looks the same as this:
Just search for “locking t-handle”. They are commonly used on the back windows of truck bed caps. I haven’t had an issue with the single center locking point. Others have posted that they’ve had some issues with the hatch warping a bit and allowing water in the sides. You could always put one in each corner if you were concerned.