The Alexander should have come with a couple of wooden houses, well four in your case, 2 with the faulty cage and 2 with the replacement one
You could always plastikote one or two of those for Spot. In my experience Sputniks are great for hiding treats in, something to climb into to investigate, they are added interest in the cage but they don't get used for nesting in, Willow and Ebony have never used theirs even to rest in for a minute let alone sleep in lol so I wouldn't suggest making a Sputnik the main nesting house.
Having said that I wouldn't take the leif house out yet either, Spot is still settling in and gaining his confidence, taking his home away from him now could cause a major setback for him.
I'm having the same problem with Hazel, she is peeing in her nest house too, but at the moment she has a plastikoted house which came Ebonys cage that wasn't being used, and a plastikoted one that came with her cage, I bought her a leif house but haven't given it to her yet because I knew she would need training
Willow and Ebony litter trained really easily, I just put their toilets where they chose to pee but Hazel is peeing in the house which isn't big enough for a toilet, and she pees in her wheel too, dirty girl lol
As for taming, I know Spot was hand tame when you collected him, which was why I suggested letting him come to the door and climb out onto your hand of his own free will, and I still believe this is achievable, but it takes time and patience
I've used this method with both Ebony and Hazel and it's worked for them both
Willow isn't great with hands, she is tame and very interactive, she will take treats from me and comes to the cage door to see me and I can pick her up but she is obviously not comfortable with it so I have followed her lead, when she comes to the door I hold her open ball there for her which she readily climbs into, she will then happily climb out of that onto my lap and she likes to be stroked, she just doesn't want to be picks up or held
On the other hand you could use the usual taming method, wait for him to climb into a tube etc in his cage and then block his exit and lift him out, or place an exercise ball in his cage and wait for him to go in and then lift him out, once he's out of his cage you could then use the bathtub method
Whatever you decide to do though remember this is really really early days, he's only been with you for 5 days, Rome wasn't built in a day, time and patience is the key