I was slightly shocked to read the 4e version of a portable hole:
Power (At-Will): Standard Action. Place a portable hole on a wall, a floor, or a ceiling. (The surface must be flat for the item to function.) The portable hole instantly creates a 5-foot-wide, 5-foot-deep hole in that surface. With a standard action, any creature adjacent to a portable hole can pick it up, provided there are no creatures or objects inside it.
This strongly deviates from the earlier editions extradimensional space concept. Would it be reasonable to use this "instead of door" if placed against a wall? Or does the hole have a "flat bottom?" Can people on either side of the hole pick it up? What happens if it's placed over an aquifer (or muddy ground?)