Well there's your problem. You just gotta reroll it. A smaller number will probably fit.
I always wanted to see a huge battle scene, like Lord of the Rings thousands of fighters... and before they start fighting, two gigantic d20s drop from the sky and make a roll.
I'm not saying I would have preferred that to what the Dungeons and Dragons movie was, but... okay, I'm saying that.
You're gonna need about 30 of those giant D6's if you want to actually make an attack roll.
What other orientation could he possibly put it in? The dude rolled a nat 20. If he can't get it to fit after a roll like that, then there's really no hope.
It is way more funnier if you first zoom into the full compartment and then zoom out.
The next step for this is to faux paint it like stone and mount it on a sledge hammer handle.