Our local pwa rider Nicolas Goyard made up (3d printing with carbon inside) this neat little set of spacers.

marseille.glissattitude.com/windfoil/u37872s1-divers-cales-de-reglage-rake-divers-2019.html

Seems it works really well for fine tuning the mast rake. I honestly think I wouldn't see the difference but he swears (and I believe him) it's a game changer.
He recently did a 35 Vmax so I'll trust him

How do you keep them in place during assembly?

There's a rake range that is optimal in terms of getting the board from sub planing to foiling. Obviously if the mast is raked back it's going to be harder to pop the board onto the foil as the wings won't be apply the most efficient lift vector. There's is a bit of room for error though and generally if your fuse is parallel to the board you're in the ballpark. Game changer is a big call....i'd say it's fair way down the list. What's he talking, how early you can pop it onto the foil, windage minimisation while in the air?

Also you've got to be super careful putting a spacer between the top of the foil mast and the deck. If that spacer isn't actually fixed (IE epoxied) to the inside of the finbox as well as the deck if you hit something big....you probably just delaminated your deck or it won't be long till you do.