A couple of years ago I made two nice sized swim steps out of 3/8 Aluminum for my southwind daycruiser, I sold the southwind and never put them on.
I will have to get you measurements, but I want to say they are at least 18 inches wide and 12 inches or more deep. They are slotted for traction and all edges (including the slots) were rounded with a 1/4 round router bit. They are bare aluminum, so you could have them painted, annodized, or powder coated any color you want to match your boat. I was going to have them powder coated with a texture or rhino lined to help with the slipperyness. All you would have to do is buy brackets to mount on the transom.
They are not a full width swim platform, but very nice steps and would look better than a swim platform and be lighter in weight.
If you are interested I'll try and find them this weekend and send you a picture. Let me know.