it's up to you to experiment.
in general, foams are optional. they are used in few areas in a mkb: gap between plate and pcb (plate foams), between key switches/stabilizers and pcb (switch foams), between pcb and bottom case (case foams), or between top and bottom housing (force-break mod). however, you should first find out the purposes of using foams in mkb, e.g., sound dampening/enhancements, vibration reduction. these foams affect typing sound and feel. if not done right, they can introduce undesired side effects, some kits are not meant to use foams to be good, and the presence of foams kills their signature characteristics significantly.
note that the choice of material and thickness depends on the desired sound and feel, and also other components in a board, e.g., denser foams like poron can offer more dampening than eva, only use thin plate foam when you use a thick top plate.
there are modular prebuiltsthat include all pre-cut foams in the market. it's a good thing, because at least you can either use or take out any of them to get to what you want to achieve.
do not use foams that are too thick that cause undesired pressures to the pcb. it might lead to weird behaviors to a board.
And below it has quite a bit of space. I will use it for a while, get the hang of it, and see again if adding foam will help.