If you’re on a budget and want to maximise your IKEA Kallax “cubby holes”, you could do a lot worse than the modular display podium system from Wicked Brick. Not every LEGO model or diorama is big or cool enough to warrant a Wicked Brick case, but you might want to consider sub-dividing your Kallax holes to maximise the space it offers.
The podium system is fully customisable and can be easily taylored to your display needs (I’m using mine as a shrine to Batman, to celebrate “Batman Day”. I’m taking half of the unit to allow me to store an extra LEGO model or two in my LEGO Star Wars cubby hole. The possibilities, like LEGO itself, are endless.