A boat just showed up this spring in the boat yard here with an aftermarket swim platform. Unfortunately I didn't take any pictures of it, but it is very nicely done. It is a Tanzer 10.5 with a transom hung rudder which makes the swim platform challenging so it is split in two with rudder stops built into it. It is molded fiberglass with the outline matching the hull shape and a small flange where it must be bonded to the transom. Unfortunately I didn't take any pictures, it really is nicely done. If you want to put a lot of effort into making a mold and laying fiberglass, you can make it look like part of the boat.
A buddy of mine with an aluminum boat made a fold up swim platform with a massive hinge welded to the transom. It has cables to support it and two pins which pass through the platform when folded with clevis pins to keep it in place.
I just have a transom mounted stainless fold down swim ladder on my boat which is also great for loading and unloading the dinghy.