I've been wondering about this, since a character in one of my tales gets on board an LM which then (naturally!) misbehaves. I'd like to get this right, or at least feasible.
I had a look at the images provided by our good Librarian here:
http://www.shadolibrary.org/aspects/module.shtml and there are three possibilities, I think. I
don't include the nose inside that cowling, as this is where the re-entry stresses would be greatest. I wondered about the cockpit canopy sliding open as on a WW2 fighter but again this would make it a weak point. There's no sign of a side panel to be opened like one of those gullwing car doors; neither is there any obvious way of reaching a panel like that on the Moonbase launching gantry!
So I had a look at the (NASA!) Space Shuttle:
https://www.nasa.gov/sites/default/files/thumbnails/image/61a-s-0139.jpg and wondered if that was the entry hatch, below the word 'Challenger'. There is a very similar feature on the underside of the LM (in the image next to Paul and colleague). If it's a hatch, it can be reached from underneath when the lifter is on the runway, or from a transit lift on the Moonbase gantry.
Does this sound reasonable? Or does anyone have any other suggestions?