Spell Creation Rules for Enchanted Container?

What it says on the tin. I am trying to make a few spells based on extradimensional spaces and could use some guide lines.

Have you looked at the Enchanted Container spell in the Player's companion?

Yes. That’s why I quoted the spell in the title.

[quote="Demons_eye"] Yes. That's why I quoted the spell in the title. [/quote]

Right-o, just checking it wasn't a concidence.

So even more so than ever after last night I am in need for some rules as to hopefully create a rope trick or private sanctum like spell. Honestly not expecting to make something like this under a third level spell but coming to the DM with some loose numbers would be helpful.