Ohh, lots of clever people!
My cat's being a brat at the moment and breaking out mum's bedroom window through the flywire, and she's unstoppable! But that won't help you now will it?
Here's some questions and suggestions rather than answers!
Cats tend to change their routines when they are stressed or unwell, as you have noted. I understand that your cat didn't act this way last time she was sick with crystals, but just to be on the safe side, keep an eye on how frequently he goes to urinate and how much: if he's running off all the time doing not much, I would go get him looked at for a bladder infection. I *would* block access to the cupboad: if you can give him good alternatives for a little while he might just break the habit nicely! Aside from the stress of changing the house around, has anything changed? I think someone mentioned changing the litter: if you have started using new litter then I would say go back to the old stuff. Has your cat been getting less attention since you started renovating? If he's using the litterbox some of the time, it sounds like he's more using the cupboard to get your attention, so if he's getting less lovin', give him more! It's also possible that he's marking his territory: is he just urinating? This would be a bit more complicated methinks but just throwing in, cats that are spayed are less inclined to do such things, so if he is spayed then he may not be spraying and if he's not spayed then you are faced with a challenge! Also, you said your cat is 9, which is not overly old but neither is it the youngest of cats. Is it possible that he is having difficulty with incontinence and is unable to make it to the tray?
Finally, cats have a will of their own: at worst you could put the littler tray *inside* the wardrobe and thus have a hilarious tale to share with your fellow cat lovers!
Best of luck!
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...though fire may burn & flames envelop me, I will arise from the ashes...