Wow - that looks better
I'd still have a lot more substrate on the left but maybe you're waiting for the acrylic panels? Or - is that where the platform is going to go? In which case the substrate doesn't need to be much deeper really if there's a platform over it.
Tricky thing with fitting platforms in a barred cage is, as you say, it doesn't have the straight sides. So the feet push the platform way from the edge of the bars and leave a gap due to the curved cage base. Unless it's a platform with inset legs! I think the Trixie living platform works that way - the edge of the platform still extends to the bars because the legs are inset.
However you could adapt one. Getzoo make excellent platforms in all shapes and sizes at 21cm high (ie tall enough to sit above substrate level (and also at about 16cm high). They have attachable legs but presumably you could leave the back legs off and glue them on at the sides instead? Not sure how that would work!
Platforms are easier to find than shelves though. I had to get my shelf cut to size from ebay and then change the fixings. I can send you the link if you like?! It fits the plaza perfectly - she was happy to cut a bit off one end to my measurements and also cut a bit off two corners (the plaza has curved inside corners) so it fitted. The downside was the fixings it came with weren't any good - they protruded out of the bars (catch hazard) and made it impossible to get the shelf in the cage with them sticking out on three sides! But they pulled out quite easily and then it was just a case of drilling new small starter holes and using normal screws instead (with the existing provided washers).
Or - you could make knuff shelves - they used to be very popular. They just sit on the cage base and build up to a platform. Or you can cut the base out and stick legs on (Ikea Knuff magazine racks). But the small hole needs blocking off or a hamster could get stuck).
knuff hamster shelf - Google Search
Actually maybe not - they're more for straight sided tanks as well.