Here's a little more detail on the process I use on the second door. First, determine the location and height of the door and afix in place.
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I then use a black Sharpie and color in the frame so I have an exact location to open the bulkhead wall.
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I then take my drill bit and lightly start pilot marks inside the frame starting in each corner and then down the sides.
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Then it's simply drilling out and using an exacto and file to clean up the cut out. Finish off with Tamiya extra thin to smooth out the plastic and you are ready to mount your PE doors when ready.
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I hope this helps for anyone that would like to add open doors or hatches using photo etched parts.