You could try these:
1) Read the “Bubble Squeeze” section in the “Firing Schedule Tutorial” under “What’s New?” at the top left side of your screen.
2) Use “Reed Glass” (Bullseye) or “Cord Glass” (Uroboros) as the clear cap. The ridges allow the air to escape as the piece fires
3) Place small clear beeds or large clear frit around the edges under the clear cap. The center of the cap will slump first, forcing the air out to the edges.
There will almost always be some bubbles.