I was slightly shocked to read the 4e version of a portable hole:

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> 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.
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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?)