Well, first things first you want to be able to side out the junk you no longer need for relevant cards.
3x warping wail
4 x null rod
1 x Chalice
That is 8 cards right there, so a lot of room to play. Things I would consider
- 4th dismember - for obvious reasons, has other sideboard applications against other lists.
- Waste - If they are on the ghost quarter plan, this is a nice land to have a copy of in the deck. Can be totally unexpected and lead to a real blowout, and works well otherwise with crucible as they only have so many ways to deal with it. Good against bloodmoon as well.
- Extra Crucibles - For obvious reasons
- Black lotus, sol ring, mana crypt - So interestingly enough, these become really quite strong if you are in a position where you can side out your null rods. Landing a turn 1 Thought Knot is really strong in a mirror match like this one. It feels strange to say play lotus in your side, but there it is. Against other decks where you would side out these cards like most white hatebears decks, they are also very good. I think the moxes though are just not good enough so i would stick to Sol, crypt, and lotus.
- Jitte - Card is an established game winner, and if you can suddenly play it, it remains as such. Sol lands mean you can have this active as early as turn 2