Permanent is hard. A portable hole, glued to the floorboards, might be about the best you can do.
But you can do sort-of-permanent: Demiplane, the 8th-level spell, should serve you well. The portal's only open for an hour at a time, but you can re-cast to access the same extra-dimensional space if you like (PHB p.231). So the stuff is stored in a ~permanent~ extra-dimensional space, but your access portal is transient. (But that may even be better!? Just make sure to protect your wizard.)
Other options:
Mordenkainen's Magnificent Mansion will do the same thing at a lower level, but the duration's not great. (Because you can only store stuff for the spell's duration of 24 hours. But you can have a nice time doing so!)
Handy Haversacks and the like nominally fit the bill, but probably aren't what you're looking for.