DIY Cabinet for (In-Ceiling) Speaker

I recently watched Zeos’ video on rear channel speakers and his conclusion was “Bigger is better in the rear.” Also he mentioned that by laying the speaker on its back and pushing it as close to the wall as possible you can create a huge zone where the sound comes from.
In this video he was using 5.25 inch coaxial woofers with tiny boxes that are not being sold anymore. My idea was to just make them myself.

So the plan is to make the smallest possible cabinet for either a 5.25 inch car speaker or 6.5 inch in-ceiling speaker. For example the Micca M-6C 6.5" in-ceiling speakers (35$ each) seem like a great option.

Does anyone have experience in designing speaker cabinets?
Where can one find the rough outlines and equations to make sure it doesn’t sound like crap?

PS: Just mounting the speakers in the ceiling as intended is not really an option where I live since all houses are built from bricks or concrete.