Dooley dropped the actual urn.
Did you spot any urn-making items?
There's some clay at the construction site. Pick it up.
There's a pottery wheel in McKing's office! Unfair.
Make it softer. You need water.
Use it on the water cooler. Now you can use it on the pottery wheel to build an urn. Can you put the gremlins into it now?
There should be instructions how to make it magical somewhere.
In the evidence room. ?
Pick up the magical tome. Use it on the urn to make it magical.
Put the gremlins one by one into the magical urn.