Hi! Is that his potty in the front right corner? I was thinking maybe you could have a sandbath there in the form of one of those glass cookie jars. You could sink it down a bit in the substrate and they are angled so it would be very difficult to kick the sand out.
One option for the shelf/house would be this rabbit house I used until recently. It's big inside so room for a nest and a potty. It's quite tall so I had sat right on the base of the cage and put about 2" of substrate inside it, then piled the substrate up round the outside of it so it was part under the substrate. It needs a bendy bridge over the door though because the door is so big. But the bendy bridge gives access to the roof as well as making a tunnel entrance. The roof lifts off the house for emptying the potty and the roof also makes a big shelf.
It's not too expensive either - they come flatpack and you fit them together like a jigsaw so there are no nails.
https://www.amazon.co.uk/dp/B001MZUY...PHTDZ9GSMAX73Q
I found it needed to be right on the bottom of the cage as it's quite big and heavy so could fall on them if they try and tunnel under it. Your cage is taller than my old 80cm one though so you might still have room for another corner shelf or something over it. The house would virtually fill the width of the cage though with just a small space at one end, so then it would be difficult to fit a sandbath in.
ImoT had this house and recently changed it for a 3 room rodipet house, which could work out better as it has 3 small roofs that lift off. Coco has this one too.
https://www.rodipet.de/shop/haeuser/...monoporta.html
It's not cheap though - with postage it works out around £29 and apparently a standard hamster potty doesn't quite fit in it, but you could use something else for a potty. That would also go most of the width of the cage, bar about 15cm though, assuming you had it in the right back corner along the back and right side of the cage, but would leave more useable floor space than the rabbit house - still tricky to fit a sand bath in.
How about having the wheel are as a sand area? The bit that is fenced off.
Actually Charlie started putting his hoard in the potty in the rabbit house (and hence peeing on it) so maybe the room idea is best.
You could probably make a cardboard house with rooms by sticking partition walls in it
I went round the houses with things like this with Charlie's old cage. I began to think I'd be better spending the money on a new cage that had a good house with it like the Alexander or Barney, rather than spending the money on a house that took up a lot of space in the 80cm cage.
That might be an option - upgrade to a Barney cage. That would cost £50 but you'd have a good shelf and house with it
More space to put a sandbath where you want and plenty of space to put a bigger house in later even if it's only a cardboard one. He might change his habits with a cage change.