I consider Flatpak bloated. We can use bubblewrap with AppImage, but its easy to make mistakes and in Qubes we already have compartmentalization.
I don’t rule out bwrap as a good security layer if we want to use another application on the same disposable VM, but I need to think about it more carefully.
https://wiki.archlinux.org/title/Bubblewrap/Examples
Example with Firefox
bwrap \
--symlink usr/lib /lib \
--symlink usr/lib64 /lib64 \
--symlink usr/bin /bin \
--symlink usr/bin /sbin \
--ro-bind /usr/lib /usr/lib \
--ro-bind /usr/lib64 /usr/lib64 \
--ro-bind /usr/bin /usr/bin \
--ro-bind /usr/lib/firefox /usr/lib/firefox \
--ro-bind /usr/share/applications /usr/share/applications \
--ro-bind /usr/share/gtk-3.0 /usr/share/gtk-3.0 \
--ro-bind /usr/share/fontconfig /usr/share/fontconfig \
--ro-bind /usr/share/icu /usr/share/icu \
--ro-bind /usr/share/drirc.d /usr/share/drirc.d \
--ro-bind /usr/share/fonts /usr/share/fonts \
--ro-bind /usr/share/glib-2.0 /usr/share/glib-2.0 \
--ro-bind /usr/share/glvnd /usr/share/glvnd \
--ro-bind /usr/share/icons /usr/share/icons \
--ro-bind /usr/share/libdrm /usr/share/libdrm \
--ro-bind /usr/share/mime /usr/share/mime \
--ro-bind /usr/share/X11/xkb /usr/share/X11/xkb \
--ro-bind /usr/share/icons /usr/share/icons \
--ro-bind /usr/share/mime /usr/share/mime \
--ro-bind /etc/fonts /etc/fonts \
--ro-bind /etc/resolv.conf /etc/resolv.conf \
--ro-bind /usr/share/ca-certificates /usr/share/ca-certificates \
--ro-bind /etc/ssl /etc/ssl \
--ro-bind /etc/ca-certificates /etc/ca-certificates \
--dir "$XDG_RUNTIME_DIR" \
--ro-bind "$XDG_RUNTIME_DIR/pulse" "$XDG_RUNTIME_DIR/pulse" \
--ro-bind "$XDG_RUNTIME_DIR/wayland-1" "$XDG_RUNTIME_DIR/wayland-1" \
--dev /dev \
--dev-bind /dev/dri /dev/dri \
--ro-bind /sys/dev/char /sys/dev/char \
--ro-bind /sys/devices/pci0000:00 /sys/devices/pci0000:00 \
--proc /proc \
--tmpfs /tmp \
--bind /home/example/.mozilla /home/example/.mozilla \
--bind /home/example/.config/transmission /home/example/.config/transmission \
--bind /home/example/Downloads /home/example/Downloads \
--setenv HOME /home/example \
--setenv GTK_THEME Adwaita:dark \
--setenv MOZ_ENABLE_WAYLAND 1 \
--setenv PATH /usr/bin \
--hostname RESTRICTED \
--unshare-all \
--share-net \
--die-with-parent \
--new-session \
/usr/bin/firefox