Are you not a member of the Apple Developer Beta? If so, you can install a macOS Catalina VM instead!
This tutorial for installing macOS Big Sur Developer Beta using OpenCore has been adapted for Proxmox from Kholia’s OSX-KVM project and Leoyzen’s OpenCore configuration for KVM. You can get the full sourcecode on my GitHub here.
I’ll assume you already have Proxmox 6.1 or 6.2 installed. Because Big Sur is currently in closed developer beta, you need to be part of Apple’s developer program, and so you will have a real Mac available to download Big Sur and fetch the OSK key.
Your Proxmox host computer’s CPU must support SSE 4.2, so for Intel your CPU must be at least as new as Nehalem, which was the first CPU generation to bear the “Core” i5/i7 branding. Older CPUs will cause the finder to repeatedly crash after installation completes (with an Illegal Instruction exception in the graphics code).
Modern AMD CPUs also support SSE 4.2 and will work with this guide.