blow off tube question

the picture attached depicts my fermentation bottle. I use 15 gallon surplus iodine barrels to brew 10 gallon batches, and they work pretty well.

looking at the picture, you can see the barrel has 2 threaded "bungs".

i use a large rubber stopper with a blowoff hose attached in the large bung.

Heres my problem. due to the rising gasses during fermentation, the rubber stopper always works loose (rises out of the hole), so the blow off tube no longer functions.i think it has to do with the threads in the opening

I am thinking a solution would be to drill a hole in the screw in cap, and inserting the blow off tube, then screwing in the cap.

My concern is having an explosion if the gasses get trapped.

Your thoughts ??

So long as the gasses aren't trapped in you should be fine. As far as the bung is concerned, have you tried putting it in there tighter or going with a 1/2 size larger bung?

I have a few of these and use a stopper/3 piece airlock body for my blow off tube. I use a, I think, a #5 1/2 stopper and screw it into the small side tight. It threads right is. My problem it is reaaly hard to screw it out. I sometimes have to use pliers to break it loose.