That's why I wrote "virtual".
The place of combustion had no walls.
Though, the shaped charge had a nozzle-shaped volume filled with solid material used as propellant after vaporization, so it was a weird kind of SRB with combined nozzle and chamber.
Actually, a normal thermal rocket, just overturned.
And the pusher plate was a reflector part of the distributed nozzle system, reverting the gas flow. Like the turbojet reverse.
Except of the pusher plates, of course, lol.
It would receive radial hits of expanding matter, and this would kill the whole idea together with the ship.
The parabola is inside the nuke. It's the nozzle between the primer and the membrane.
The advantage of Orion is exactly in absence of radial stress limiting the engine size and power.
That's why the Mini-Mag Orion should anyway have no chamber, but should ignite the pellets with X-ray emitters far behind the ship, outside of its structures.
And that's why irl the Mini-Mag "pellets" would have complex structure, not simpler than the classic Orion nukes one.
They anyway would need heavy metal as final propellant, to carry momentum in dedicated direction.
Just without fissiles. (And it's a question if it's easier to make such Z-pinch portable machine or to produce twenty tonnes of oralloy more.)
Also, when the D-D fusion reactors working on oceanic deuterium will be usual, the oceanic uranium will be being extracted and enriched as well.
So, the unlimited power of D-D and unlimited oceanic uranium can easily make oralloy and tritium production by orders of magnitude higher, and fission Orions normal, due to their simplicity compared to remote fusion.