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I first used the fuselage side panels to draw the shape of my panel. I cut it out on my band saw and then filed it down to shape. I left a gap of aproximately .032 all a way around the side panel opening and my access panel edge to allow for paint thickness.
Next it was time to make the mounting flange. I found a large washer that had a width of 5/8" from the center hole to the edge. I used this to trace an outline around my panel. I placed my access panel on top of the material I was making my flange from and placed the washer directly up against the edge of it. I simply used a Sharpie pen and placed it in the washer hole and rotated the washer all a way around the edge of my access panel to get an outline of what will be the outside edge of my mounting flange. Then I cut this out and filed the edge smooth.
To get the outline of the inside edge of the mounting flange I measured in from the outside edge 1.25" (2 x 5/8") and drew lines that intersected on all four sides. I then drew rounded corners to my liking. I drilled holes near these corners to provide access for my jigsaw blade and carefully cut the inside out.
Filing the inside is a challenge and requires patients to get a nice finish. On my second one (I have panels on both sides), I used a hole sander in the corners first to remove most of the material and then finished with the file.
The next step will be to mate everything.