You'll notice there are only nine rows (If I'm counting right). Original game had an optional 'bonus bug' rule. If you roll 0,0, extra bug.
My board had a short rail around the hole; give this a thought? Also, I wanted to cut out the hole for a dice rolling tray, but never got around to it.
Well done! I used to be locally famous for running this; usually had one player per marine, in spite of the rules making it perfectly possible for up to four marines to incapacitate/die before taking any actions. Death-at-start only happened a couple of times, but is a bitter pill.
Mind you, nine players plus me rolling for the Aliens and inforcing my own 'bonus bug' rule to keep table talk to a minimum could get pretty crowded.
Doug