My pulse-jet is a tube with two narrow points.
In each of these narrow points there is a cone-shaped plug to seal it.
The plugs are kept loose inside cages that are affixed to the tube to keep the plugs in position and oriented to seal the narrow points without blocking the airflow much.
On plug(interior) is in the space between the two narrow points, one(exterior) is outside of that space.
When the explosion happens, both plugs are pushed out. The interior plug is pushed into the narrow point and seals it, while the exterior plug is pushed away from its narrow point and stopped by its cage, which opens that narrow point fully and allows the air out.
When the explosion recedes, both plugs are pulled in. The interior opens and the exterior seals.
In this way, which of the narrow "necks" of the tube is sealed alternates based upon whether the interior is exploding or imploding. Thus it alternates between pushing out of one opening and pulling into the other.
That said, it is just a shaped tube with a couple of loose bits and repeated explosions. With our crystal shaping and explosion magic and repeat-casting fogs and curuits it should be very easy to make. The only difficult part is tweaking the relatively constant continuous fog into a rapid repetition(which I suspect that it already is) and apply it to the explosions. But we got the continuous spell from tweaking an existing spell already, so doing a very similar thing to a different spell should be relatively easy. I really have a hard time figuring out where extra difficulty could come from.
Add to that that it is pretty plausible too. The idea of using explosions as propulsion is fairly plausble. After that it is just a matter of realising that it pulls after it pushes, so you need to force it to pull from somewhere other than where it pushes. This gives you the idea of a manual plug, and seeing how it automatically pushes the plug out, it is not far to realise that you can use that force to make the plug automatic. I half-think that we could make the thing from an order if we were willing to force an apprentice to keep tapping the thing for a rate of explosion that is too low to get the most out of it.
...
Apparently real pulse-jets don't bother putting valves on the outflow.I guess the blockage would be too much and the outflow is strong enough to impose the suction ipon the newly opened vent rather than the massive opening that never closes. Also they are kind of weak compared to jet engines, ore even piston engine, maybe, I didn;t look too closely, but they were enough to keep V-1 cruise missiles airborne for an hour after being launched by a catapult, so they can fly while lugging a massive bomb with tiny wings around at least...
https://en.wikipedia.org/wiki/PulsejetIt is also interesting to note the pulse jets that have no valves, just two holes of different size and length, but I can't think of a good reason why we would create those, and the vlaves should be pretty easy.