While the "dimensional" critters idea is very reasonable, I would go a different way. Create a recipe, that will require several steps and tasks to gather.
Find a type of tree with very deep roots. Something that sends it's roots primarily downward, with little branching outward. The tips of such a tree are required.
Then, decide how the quiver works in terms of how it makes an inter-dimensional quiver.
Is the opening a constantly functioning teleportation portal to several large quivers? Find an ingredient that speaks to interconnectedness, especially over surprising distance or at surprising scale. Fungi that helps trees communicate. Whale parts. Part of a telepathic creature.
Is it still a single object but magically enlarged on the interior (ie bigger on the inside)? There are many creatures with impossible dimensions in dnd, pick one. Or, pick something that starts extremely small and grows to incredible size, but you've already got tree roots in the mix.
Of course, this only really works if your players enjoy weird complexity.