Okay, so me and another player in my campaign have begun to make out own loot stash together (we're both Kobolds). We are both wizards (Me an Onomancer and them a Lore) and we think we have come up with a pretty decent method for hiding a 5x5 cubby in our base.

We dug it into a wall, lined the interior with a thin sheet of lead (against detect magic). He hid it (DC 18 investigation to find). Finally, we crafted a Lock of Trickery and arcane locked it to get a DC 25 for picking it. We then used Nystul's Magic Aura to cloak the magic from the Lock of Trickery and Arcane Lock. We also have Private Sanctum to prevent any means of getting inside and seeing inside. On my last level up, I acquired Faithful Hound to alert us to anybody coming close to it as well. Leaving us with this:

Secret Cubby (DC 18 investigation to find)
Lead (Against Detect Magic)
Lock of Trickery (Dis on picking)
Arcane Lock (DC 25 to pick)
Arcanist's Magic Aura (Detect Magic can't see lock)
Private Sanctum (Blocks all attempts to access through planar travel and divination magic doesn't work)
Faithful Hound (Detects all people who don't speak password regardless of sneak, planar travel, and invisibility. Also attacks hostile creatures within 5ft with a +7 to hit and 4d6 dmg)

We plan on getting Glyph of Warding to summon Elementals as well later when it's touched when either of us are around.

This leaves me with this question... What else could we do to improve this?

What’s the door/hatch made out of? I can see a high STR character (maybe with a guidance) just bashing it open with an axe.

Demiplane.

Problem solved.

The door made out of stone

So bashing would be hard, but still an option. Maybe line it with something with a high hit point value.