Set o-ring into groove rtv or high tack it in place.

place the o-ring in a groove. Far better sealing can be obtained by using two O-rings either together or in seperate gooves. the grooves need to be about 2/3 the o-ring diameter in depth so they squeeze into square setion when compressed but do not foul the opening.

Need more informations.

How is it sealed now?

What is air quality in the tank?

How the door is closed?

Silicon o-ring seems to be a good solution. There is also braided packing with elastomeric core which may be useful for you. Later I'll find a link.

---best.czes

In addition to RTV-ing the o-ring into the groove, you can add vacuum grease to the mating surface to help make a seal and to prevent the o-ring from sticking and becoming damaged upon opening. This is especially helpful if the process requires opening the door on a regular basis.