Getzoo sell brilliant platforms. I bought one for a tank. They do different sizes and different heights. I got the taller 21cm tall one - so you could have a good depth of bedding under the platform and still have space under it. I didn't find ladders worked well with them, so used a large cork log from Rodipet partly under it which made a ramp up and a tunnel under the platform as well
They even do one large corner one that fits on virtually two full sides of a cage. You'll notice each platform listed more than once - because one is 14cm high and the other is 21cm high (14cm isn't really high enough to sit over substrate).
https://www.getzoo.de/einrichtung-zubehoer/etagen
However, they are just pieces of wood with legs - so probably easy to make! The legs on these slot into place which makes it easier.
Sorry - I cant remember if you're in the Uk or not! If the Getzoo ones aren't available to you, they might give you some ideas for making one - eg shapes, corner platforms, heights etc.