My Phone case melted

Evgeni, I'm confused by your question. According to the repair guides, the rear case of the OnePlus Nord is glass, and we already know the front screen is glass, so what exactly melted? Are you saying the "heat" your phone was exposed to was enough to melt glass???

Okay, with the pictures it's starting to make sense. The part that melted is the midframe, or rather the molded plastic surrounding the metal of the midframe. Regardless, you obviously need to start with a new midframe. I managed to use the AC2003 model information on the iFixit device page to locate a midframe that could be used. Note that it comes in green or black; from your pictures I assume the black one is correct for your phone.

Oneplus 8 Middle Frame

In addition, you need a rear case; here are a couple that look right.

One Plus Nord Back Battery Cover

It's not clear whether your screen is still functional or not, but it does appear there's been some heat damage on the left side and bottom edge, so I assume you'll probably need to replace that as well. Since you're going all in on the phone anyway, a new battery would probably be wise; they don't handle being exposed to heat very well so I'd suggest doing that too.

There don't seem to be an guides available for replacing the entire midframe, unfortunately. I'd suggest you use the Teardown as a template and see if you can use one of the available guides for the individual steps.

OnePlus Nord Teardown - iFixit

Hopefully that will get you going; if you feel ambitious you could always author your own guide here on iFixit to help out anyone who comes along after you with the same problem!