I just want this for linux educational purposes, I want to be able to figure out how to do something like use a windows manager as a desktop environment.
Create a standalone HVM by installing from an iso - that will run in a
window and you can install window managers as you choose.
You can also try running a nested X server. You should be able to run another window manager within it. See the ArchWiki for more details: Xephyr - ArchWiki This might be easier than installing an HVM and you will still have access to all Qubes tools (qvm-copy, etc.) without going through extra steps.
What I would do:
1- Install VirtualBox or VMWare (free version) on my main OS (Windows, Ubuntu, etc.). Preferably on a laptop with Hi-Speed USB 2 or even faster/better SuperSpeed USB 3 - to be able to save VMs on a USB Flash Drive or USB Disk and run different instances of VMs from.
3- Install a main/template VM using the ISO. Start the VM and make sure everything is working. Install programs I want to have and update the running OS (inside VM). Shut down the OS (VM with saved changes).
4- Make a copy of the template VM, name it like OS_VER_WM (Ubuntu_20.04_ Fvwm2 or Ubu_20.04_Fv2). Run the copy. Best to get/make a new IP/MAC-address so I can run different sessions/VMs simultaneously.
5- (Fvwm2) Run the new VM (Ubu_20.04_Fv2). Setup Fvwm2 and make sure everything is working. Save and exit, shut down.
Repeat for another Window Manager or other purposes:
(4- above) Make another copy of the template, but make sure the template is updated before making a new copy (if I made the template some time ago, like yesterday )
(5-above) Setup new Window Manager or whatever purpose that makes it different from other saved VMs.
Then I can use my USB device with saved VMs connecting to my laptop or PC or which physical machine I want and run them as long as it also have the VirtualBox or VMWare or whatever virtualization product I made them with.