I wasn't aware of this nifty item from Muppets from Space until today. Here it is in GURPS DF terms and generically DnDish terms, as well.
GURPS Dungeon Fantasy
Door In A Jar (Grenade). This jar of greenish slime is thrown against a wall. It creates a door, 7' tall by 3' wide in a wall up to 4 yards thick. The door lasts for 10 seconds, and allows travels in both directions. It is otherwise identical to the Create Door spell. $3,000, 1 lb.
Generically D&D compatible
Door in a Jar. This jar of greenish slime is thrown against a wall. It creates a door, 7' tall by 3' wide in a wall up to 10' thick. The door lasts for 1 round, and allows travel in both directions. The door does not actual create a hole, but rather a two-way short-ranged gate that bypasses the material between the two points. 750 gp, 500 xp.
Why so expensive in DF, and cheap in D&D? In both, I put it on par with Flight/Flying - potions which otherwise bypass obstacles that are at the heart of dungeon delving. DF puts a high premium cost on things that do so; D&D cares a lot less about this. So the pricing is in line with how both games value such things.
Why not just make it a liquid version of Passwall? Go right ahead, but I like the effects of Create Door a lot so I went that route.