With the recent increase in "slot top" cabin boat construction there seems to be more and more inquiries about how to cover that open slot against the elements. Hard covers present the challenge of design, construction, fitting, securing and storage. Way too much thinking and work for me. Soft covers offer a cheap, easy solution. A small section of any waterproof tarp will do. Heck you can even buy a new polytarp large enough to cover the slot for less than $10. OK, so you have a tarp, you cut it to size (make it much wider than the slot, best to make it nearly as wide as the cabintop to keep rain from blowing under it), and you have grommets installed around the tarp perimeter. Now you need to make some suitable attachments like eye pads (or a handrail) to the outer edge of the cabintop. Might as well make these substantial enough to use as tie downs for oars, boom, mast or whatever you might want to lash to the cabintop when spending some time at anchor. When aboard, run small line from the tarp grommets outboard to the eye pads and back under the tarp into the cabin so you can stretch the tarp tight from inside the cabin. Now all is well until it rains and your tarp sags from the weight of the water it collects... Here's an easy solution to that problem... good old 1/2" pvc pipe and fittings. Cut the top off of some T's and glue them to some pipe with a 45 degree elbow in between and you'll have some pressure fit rafters for just a few cents each.. Easier to describe these with photos than words so look here.
Oh, while you're at it, make a single piece clear acrylic dropboard for each companionway hatch, make the tarp long enough to overhang them and you have a cosy slot top cover on the cheap.