I was working on a one stick with integrated platform before I decided to try a ROS system. I made my platform of plywood, fiberglass, and carbon fiber laminations. I just looked at a couple of commercial platform designs and slapped it together. Not a plan, per se, but pretty easy to look at and decide whether you want to try it. I'm happy to answer specific questions.
Original plans (shows 2 different size options) intended to be from cast aluminum. Dimensions are about 14 inches wide by 10 inches deep for the larger design. (I have size 13 boots)
Original design (slightly modified-thicker webbing- for wood) laid out on plywood with first layer of fiberglass epoxied on (still wet).
First layer of carbon fiber. Weave offset 90 degrees from fiberglass. (Final lam was a layer of fiberglass and 2 layers of CF on each face of the wood.)
Below is the full stick, for reference. In this picture you can also see how the CF and fiberglass wraps from the flat to the vertical panel, providing a lot of extra strength and rigidity. This is repeated on the backside of the platform so you have the same rigidity no matter which side of the platform you put pressure.
For just a platform, I'd probably add two 6-8 inch long vertical posts offset from centerline to give more lateral stability. You could very easily make wedge shaped posts with holes cut through for a strap.
Weight reduction holes drilled. The whole stick weighs just over 2 pounds with the platform. Platform by itself is about 1 1/4 pounds.
One more pic, just to show the overall outline of the platform top.
I would recommend adding some sort of metal teeth to give more longevity to the whole thing. It would be very easy to put them on the vertical post(s) to give you yet another lateral and vertical "double" stability. I've only had it in a tree 4 or 5 times so far and haven't seen any significant wear but, I suspect, even with the CF and fiberglass the teeth on this one would wear out in a season without some reinforcement.
Generally speaking, I like the platform a lot. The final dimensions ended up being about 12 inches across the "front" of the platform, 8 inches at the tree and about 10 inches deep. It's big enough for my feet without being "huge".
I may modify this to remove the steps and just make it into a platform but, for now, I'm just trying the ROS with strap on steps and then figure out what my final system will be.