The maths in old money was easier numbers for me like
never really worked in imperial before ... but small numbers + tired bekki = easier than bigger numbers with decimal places using metric!
You will need strong correx for the bottom if not using grids
5mm thick at least id say.
You dont even need to make a top level really if you dont want to and then you could keep it simple and have a 6x2ft cage with 1ft walls and no grids and it would work out cheaper that way
heres a video showing mine
its a bit shaky but you can see it quite well
New Guinea pig correx/corotherm/coroplast only C&C cage and room update! - YouTube
If you have already got grids id suggest using them as it makes hanging water bottles and making it nice and strong easier especially if you are using thinner correx and are weary of escapeees! I can imagine some pigs will be able to escape 10 inches especially if they have plastic houses and stuff in the cage they can jump on first mine never escaped the 12 inch walls though but they were old lazy girls