I don't think you can make a good shield-less broadsider in the vanilla grid. This is the closest I came, but I think the cannons need to be spread further so you have more armor surface area on each side. Then have some kind of spinal in a longer nose section.
The thing that hurts a lot when fighting it against a shielded broadsider is that if you spray cannon shells across the entire side, the shielded ship doesn't feel anything while the shield-less takes heavy damage. With 5 tiles between cannons instead of 3, you should take twice as many of the hits to armor and you can use the longer diagonal armor wedge so your PD has better fov. More like: