The kernel instructions to compile your own kernel at https://www.qubes-os.org/doc/managing-vm-kernels/#installing-different-vm-kernel-based-on-dom0-kernel seem incomplete, so I'm making a slightly more complete version here:

1. do:

   sudo qubes-dom0-update qubes-kernel-vm-support kernel-devel
   

2. Find a valid version (which will eventually be used as a parameter for qubes-prepare-vm-kernel) by doing:

   ls /usr/src/kernels/
   

   each subdirectory name should be a valid version, and will be a valid parameter for qubes-prepare-vm-kernel.
3. Pick a subdirectory / version and enter that subdirectory. Example:

   cd 5.10.61-1.fc32.qubes.x86_64
   

4. Configure the kernel by editing the .config file in that directory (note, you cannot use "make menuconfig" or "make config", unless you install the special dependencies for those) (Also note, there are a lot of options, so it may take a while)
5. build it with qubes-prepare-vm-kernel {name of exact subdirectory you chose}. Example:

   sudo qubes-prepare-vm-kernel 5.10.61-1.fc32.qubes.x86_64
   

   This should succeed in placing a new copy of the kernel in /var/lib/qubes/vm-kernels/{version of what you just configured}

However, the compilation process runs way too fast, so it is obviously not compiling the kernel. Possibly just managing modules?

Confirmed: typing make all or make zImage returns "flex: command not found", so it is not capable of recompiling it as it would require packages to do the actual build that are not on your system