Yeah, I just went through the same custom installation process on my boat, and I can tell you that the Krogen Manatee was never intended to have a swim platform.
First, be familiar with the shape and construction of your transom. Is it cored? Where? Is the thickness uniform throughout or is it like a mountain range. Soft spots?
If you've got a thru-transom exhaust, it's beneficial to protect its location if you like it where it is. If it needs raised or relocated to a side exit, now is the time. The platform either has to clear the exhaust by its height or by whatever design needed to provide for its uninhibited flow. I moved my genset from amidships to the veranda and chose to exhaust out the transom as well, so the mod required the same thinking.
Next is to plan the bracket and touch-point locations instead of leaving it to the design of the platform. Most transoms, maybe all transoms are not perfect. The same bracket mounted in one place is an inch lower at the tip when located on the opposite side. Don't end up trying to bush the bracket out with spacers, wedges or washers to make it level. The transom will only be happy with a flat fit. If you have to, space it under the platform but not against the transom. It took three more 25 mile trips to the welding shop to make mine that way and it was well worth it. The location of equipment in your lazarette, transom shape, stringer location, etc. can really ruin your day when it's not planned for when mounting brackets. My transom lifts out of the water on both sides and required six brackets below and two above to equally secure it across its width. If there is anywhere on the platform that unsupported leverage can't be transferred easily to a bracket point, you'll have to make up the difference in thickness or design of the platform and/or material.
I'd say that weight should be considered, but your boat looks like it could swallow up a big platform without much concern unless you plan on moving something else heavy aft. Still, it helps to know the your immersion formula for your boat. Moving my genset aft, adding the platform and brackets, and then a bigger ladder, and maybe a heavier outboard, all of the sudden I'm an inch down in the transom and an inch up in the bow. You get the idea.
I ended up with 8" above the waterline. This seemed to be a good compromise for exhaust clearance, ease of access to the water and dinghy, and still left no substantial step down from my veranda deck height, maybe 1.5 inches. How much your boat squats at what speed might be an issue, so you need to know that too. At 7.5 knots, my platform is getting wet. At 8.5, it may as well be a huge trim tab. When anchored, it takes a pretty steep wake to catch it. The boat simply lifts up before much gets to it. If your transom gets slapped by a wake more than it lifts over it, you may want a higher mount.
The rest of it is choosing the material, trying not to make your platform an obstacle in handling, and to make it something attractive or fitting to your vessel. Best of luck.
I even have the hinges for that. Don't know if my backyard eyeball engineering is up to the task or not.
