There are any number of small hatches. Which you could put on the roof. Remember that the roof is crowned--you can build a flat surface, as I have done for the air conditioner--and put a hatch on that. (Build up with resin and glass, keep fair and level.
There is a low profile hatch by Lewmar which is about 10 1/4 x 10 1/4. That is a good size. You want ventilation, but not a lot of sun coming in. If you had hatches, you might want a shade under them. They collect a lot more heat than the cored cabin top. I had a sky light in my motor home--and it had two shades under it--one translucent, and one which was light occlusive. It was double layer acrylic; air space to give some insulation.
In several of my sailboats which had pilot houses, I put in the small hatches as the above Lewmar. One faced forward, and one aft. That gave ventilation, and a good way to see the sails from the inside station. Those boats were sailed in the tropics extensively.
Opening the front window and side windows gives a good air flow. You need to run the boat before starting to cut. Dorades don't give a lot of air flow. They are designed for ventilation, where there is serious water--so that the water is trapped and drained off, where as the air goes thru the baffles. They usually have a shutter in the bottom port where they come into the cabin, so the air flow can be totally cut off. Fit and used well on sailboat boats--I found the small hatches were far better than the dorades for circulation. By the time the air flows thru the baffle system, there is not a not of velocity.
