One of my lava lamps was missing its top cap. Measured and modeled this based upon the cap on my other lamp.

I printed mine in PETG which has proven durable in the high heat of the lamp, but PLA might work as well. If you give it a shot, let me know in to comments what its durability is when printed with PLA.

This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/i4fyjncxF4C