No, same litter, nothing changed in the house (apart from us being in more due to lockdown!) Have tried enclosed ones before, but the stink of ammonia after even one wee is awful and not surprisingly they refused to go in it. The vet has checked him over recently (he also brings up his food on a fairly regular basis due to bolting it too fast, or nibbling plants round the house before we can stop him) and says there is nothing wrong with him. I'm guessing behavioural specialists would cost a fortune.
I think the answer is probably an even higher tray made out of a storage box as you suggest, the one he has I would have thought would have defeated him, but I guess he sees it as a challenge! Currently I have a sheet of foil blue-tacked to the wall from high up, finishing in the litter tray, which has been okay for 24 hours, but we are going away for a week, leaving the two terrors in the charge of our DiL, so goodness knows what we will come back to.
Just wondered, if I got a very low edged tray, would that change the angle and keep it inside? Grrrrrr!