Minimal Disposable Firewall Template: Can't make template disposable

I’ve blown too much time on this. I’m sure the answer is simple for the pros.

I’m trying to make a min disposable template for the firewall.
I’m following Sven’s guide here:

I’ve made the deb-11-firewall template, and installed the firewall packages: qubes-core-agent-dom0-updates, qubes-core-agent-networking.

I’m trying to make it disposable. I get the message:
Invalid property 'template_for_dispvms' of deb-11-firewall

I clone the template to deb-11-firewall-dvm and try to make it disposable. I get the same error.

I try to set either as the template for sys-firewall. Only the disposable templates are allowed as options.

I try to set it thru command line:
[user@dom0 ~]$ qvm-prefs deb-11-firewall template_for_dispvms True
I get:
[user@dom0 ~]$ no such property: ‘template_for_dispvms’

I’m able to make it an AppVM or a TemplateVM. When trying to make it disposable, I only get the already disposable templates.

I can imagine maybe I have to create the base template using a different command than:
sudo qubes-dom0-update qubes-template-<DISTRO_NAME>-<RELEASE_NUMBER>-minimal
Searching thru the docs, I haven’t found the answer.

Is it not possible to generate a minimal template that is disposable?

to create disp firewall, you need to create appvm, then tick disposable template or set via qvm-prefs.

this is what they say as DisposableVM, then from here you can create DVM Based on this appvm, it is how it work.