The jetspacks from simply jetpacks can't have airtight seal added to them in the chemical reactor as there is no recipe. Since airtight seal is an enchant, a recipe could be added to allow have airtight seal on the jetpacks.

I'm currently using 5.9.2.

The enchant can be placed on in an Anvil.

This is in 5.9.3, please update and feedback findings.