I'd like to create a dimensional pocket inside my armored wagon. Duration should be permanent, but if that's not possible longer lasting is better. It would be fine if I have to refuel it every so often.
Can wizards go about doing this, or is it up to DM fiat now?
I'm used to being able to do this in older editions through magic item creation or the spell Genesis.
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.
