Can I get some feedback regarding a magic item I’d like to create?

Can I get some feedback regarding a magic item I’d like to create?

Can I get some feedback regarding a magic item I’d like to create?

So, one of my PCs is playing a bard and I’m pretty sure he’s modeled off of the Mariachi for Desperado (has a guitar case full of guns). I’d like to give him something similar, maybe a guitar case that lets you pull out a random weapon, depending on what you roll.

Here’s what I have:

From the outside, the guitar case of the famed musician, [insert name] looks like a normal, beat-up guitar case. When opened, everyone but the owner sees an acoustic guitar. The owner, however, sees a shimmering portal. When the player needs a specific weapon, he can reach into the guitar case and pull out whatever the situation requires.

10+: Pull out exactly what you were looking for

7-9: Pull out a weapon, but not what you needed

-6: Pull out something completely useless, oftentimes, not a weapon at all.

So, its basically a bag of holding but with a different flavor. I’d appreciate any ideas or tips anyone can share. I really want the PC to love this thing.

Thanks!