Wanderer Vagabond

The guy on the mooring next to us in Hawne Basin has pretty much done what you are proposing. The boxes in the photo don't have a solid top, they are canvas tops held on by bungee cords, it was to give you some idea of just how high you can get away with on top boxes. The only place I can remember having had to take them inside was to get through Standedge Tunnel, so if you are planning to go through there you'll need to make them removable. I haven't got through Froghall Tunnel yet, and I suspect that I'd have to take them off to do that, but for all other canal bridges and tunnels they have been perfectly fine. ETA This is assuming that your boat isn't a lot higher than most narrow boats

I have seen this used idea quite regularly(not that I have solar panels) and you can get away with quite a reasonable height. This is an old picture of my boat and you can see, compared to the mushrooms the top of the covering on the boxes is quite high. Bear in mind that every time I go out I have to get through Gosty Tunnel