No need to drill a hole. The stopper around Immersion’s rod naturally out-gasses when the internal pressure reaches 2.75psi. You will hear the slightest “pfffft” every few minutes coming from the stopper. The only consideration at that point would be overactive krausen, but since you’re keeping your temperature in check with Immersion, that is far less of an issue. Also, the domed lid dramatically increases head space further reducing that risk.
If you still want to drill a hole for air release, the sleeved grommet that came with your Immersion requires a 0.5" hole. We use step bits to drill on stainless and it’s pretty easy.