And, again, how many times do you need to change the size of the entrance to your extradimensional space? If a 1' diameter hole is too small, a 2' diameter hole on a tower shield should work.
This is only a good idea if you assume that the space inside a Portable Hole generates its own gravity towards the bottom of the hole rather than being affected by the dominant gravity of the area in which it's set up. If you assume that it generates its own gravity and that it can be set on a vertical surface (which is a necessary prerequisite assumption to putting it on the back of your shield), a portable hole becomes an easy and cheap reverse-gravity-hallway trap, which seems unlikely to be the intent given that Reverse Gravity is not on its list of requirements.
Given the lack of Reverse Gravity in a hole's requirements, it seems likely that a Portable Hole (and thus an Enveloping Pit) assumes the gravity of the area where the hole is set up. This makes building your tower in one and then setting it up sideways a terrible idea, unless you have Krazy Glued everything in your tower in place.
I see two options for dealing with this: a custom Portable Hole involving gravity magic of some kind (in which case you're dealing with the custom item creation rules anyway) or only set up your hole on a horizontal surface, which precludes using a tower shield.