No, I wouldn't call them aesthetically pleasing at all.

But you are the only one that will care, your snake will be happy with a secure-feeling "cave", & you can put some foliage over the top anyway to camouflage it- black tends to just become a shadow that doesn't stand out much. What's most important is making a snake feel safe & secure, so they eat & thrive.
As far as converting inches to cm, just google "convert cm to inches" & you'll have a easy-to-use tool come up; you can plug in various measurements to see what size is going to work for your pet.
A little too big won't hurt, you can always wad up some paper or more substrate inside it to make it temporarily more cozy while he grows into it. OK?

I'd probably get 2 mediums (@ Reptile Basics
https://www.reptilebasics.com/medium-hide-box) for him right now- from what you said, he's about 20" long, so curled up, he'd have plenty of room for a while in their size medium. It would probably save $ in the long run (on shipping) to get a couple large ones too, for him to grow into- at least, that's the way I do things- plan ahead- snakes do grow.
I don't think in C* temperatures either, so I do the same thing when some threads ask about centigrade temps. (just google "convert C* to F*- SO easy!)
Don't worry, nobody knows everything, not even me. That's why we share our experience here, so we all get smarter & find good solutions. You will too, in time. If there's something you don't understand, just ask.