Get started (for both)
First, go buy some wood boards.
Kidding, first read this whole post before buying anything and get how much you calculated you’ll need times 1.5. You’ll want scrap and extra for when you mess up. As for what to use, it’s up to you but don’t use crap. Get some Oak, Cedar, Alder, something like that. Mahogany is my go-to. It looks awesome, generally has great grain, and is super dense. It feels really good to hold, though is pricey – you can expect to spend $50-75 just on the wood for a 12″x12″ enclosure. Your local lumber yard is your friend and see what they have and recommend.
Once wood has been acquired, cut the wood. I find it best to actually measure against the metal plate I’m using instead of a ruler or something and then, add the thickness of the wood times 2 for all 4 pieces for a Pretty One™. That would mean four 13″ planks for a 12″ square metal sheet and 1/2″ thick wood.
For a Rustic™, cut two pieces that are the same length as your metal sheet and then two that are sheet length plus 2 times thickness. For a 12″ square metal sheet, you’re looking at two 12″ planks and two 13″ planks, each being 1/2″ thick.
I’d also recommend adding 1-2mm of wiggle room. It’s better to cut slightly too long than too short. Sanding down wood a few millimeters is easier than filing metal. A millimeter or two of slack is also not noticeable when the metal sheet is siting upon it’s wooden throne.
The height of the planks is all preference, unless you have components that will be mounted inside that require clearance. That said, I like a slightly tall skirt, though 2 1/2″ up to 5″ is a good height. I like 5″ personally. 1/2″ is the minimum thickness, though 3/4″ is better. Length is entirely up to how large the square hold needs to be.