Yeah, that's a good question. I haven't given any advice as to how to set up a mental compartment, and I doubt it's the sort of advice you're going to find around these parts :-)

Setting up a mental compartment is easier than it looks.

First, pick the idea that you want to "believe" in the compartment.

Second, look for justifications for the idea and evidence for the idea. This should be easy, because your brain is very good at justifying things. It doesn't matter if the evidence is weak, just pour it in there: don't treat it as weak probabilistic evidence, treat it as "tiny facts".

It's very important that, during this process, you ignore all counter-evidence. Pick and choose what you listen to. If you've been a rationalist for a while, this may sound difficult, but it's actually easy. You're brain is very good at reading counter-evidence and disregarding it offhand if it doesn't agree with what you "know". Fuel that confirmation bias.

Proceed to regulate information intake into the compartment. If you're trying to build up "Nothing is Beyond My Grasp", then every time that you succeed at something, feed that pride and success into the compartment. Every time you fail, though, simply remind yourself that you knew it was a compartment, and this isn't too surprising, and don't let the compartment update.

Before long, you'll have this discontinuous belief that's completely out of sync with reality.