For your reference, here are a couple of pictures of mine that should be easier to work from than the ones from the
GW website. You can see what chromedog means with the irregular octagon, especially with the open section. If you hold it square to you, the N/E/S/W sides are 3 sections large, and the NW/
SW/NE/
SW (diagonal) sides are 2 sections large.
Incidentally, there is a Mantic landing pad that is a decent proxy (especially for non-imperium). But if you're going to be playing
IoM, this is actually a really big model -- it's deceptively large, that is, until you have to transport it or fit it into a case. The swinging doors even actually work. For the price, it's pretty cheap.