Twenty should be spelled out instead of written as a numeral.
The comma after into should be deleted.
I made a couple of other changes to improve the flow of the sentence (changed you are to you’re and deleted that), but they’re just recommendations.
About twenty minutes after the oil drum you’re crammed into was filled with concrete, you realize you’re probably immortal.
If this example wasn’t written in present tense, it would flow even better. And what the hell, let’s improve it further by changing it to first person instead of second: About twenty minutes after the oil drum I was crammed into was filled with concrete, I realized I was probably immortal.