I know it’s old tech, but an elastic band on a couple of hooks or whatever can be quite easy to arrange on something like this with loads of room.
A couple of my boats use his, as decent size magnets would have narrowed the access hatch. A bit of bet wire can be handy to hook bands up if needed, but shouldn’t be an issue with yours.
A couple of bits of foam good idea to keep the top floating ifall else fails!
Ashley