Try the smaller hatch
Our 285 has a smaller hatch just behind the mast and it seems to be at the high point of the cabin. It offers some venting when it's open, especially when we're cooking in the galley, and it might make a good location for your solar vent if it can fit there. We have a solar vent (that has a lithium battery in it for night use) in the center of our forward hatch with vents in the top wash board at out companion way. The only few reasons that I can't think of not to mount the vent in the samller hatch (if your boat has one) would be if the vent is just too big, when the vent is open it gets in the way of the boom vang (if you have one or are thinking of getting one), or you want the air flow to go from the rear of the cabin all the way to the foward hatch. The last reason would be the best to mount it foward. If it's off center would that be so bad? The call is yours.