The basic idea was to scratch-build a cover similar to those available by 3rd party vendors such as Airward. A while back, I had made the 7" x 11" cutout in the R forward fuse rib, and rough-cut the doubler plate that fits behind it. I didn't have any 0.040" aluminum sheet handy at the time, so I couldn't make the cover itself (I had made the doubler out of thicker 0.050"). I shelved the mini-project until now, since I do have the necessary parts to continue.
First step was to trace out and cut a piece of 0.040" sheet to make the cover itself. After deburring, a quick test-fit seemed to look good and I was then able to lay it on top of the doubler plate (which had not yet been cut out in the middle) and start match-drilling 20x #40 holes between the cover and the doubler. I did this by drawing reference lines 1/4" from the edge of the cover on all 4 sides, and marking the 4 intersecting points with a center punch. I drilled those 4 corner holes #40 and then used a rivet spacer fan to mark the remaining holes, then drilled them out as well. Once all 20 holes had been drilled, I took the cover and final-drilled those holes to #27 (for #6 screw) and then dimpled the holes.
The tunnel access cover after drilling & dimpling
Next, the doubler had to be drilled for nutplates. I clecoed a #6 nutplate to the holes that were match-drilled in the previous steps, and then match-drilled the nutplate on to the doubler in all 20 locations. After all this was done, I could now enlarge the #40 screw holes to their final #27 size, and then countersink everything using #40 and #27 countersinks and the microstop. Once all the countersinking was done, I drew lines around the inside perimeter of where the cutout should be, drilled 1/4" holes at the corners (to radius the corners) and cut the rest out using a jigsaw. After a bit of cleanup with the file, I had this piece:
The tunnel access doubler
Both pieces as they will fit into the tunnel
Looks good! In hindsight, I'm not sure if this was worth all the extra trouble over just buying the darn kit for $60. There's probably about $20 of raw materials here if you count rivets, screws, nutplates, and sheet metal; plus several hours of labor fabricating everything. The quote "There are two kinds of people in this world: People who spend their time saving money, and people who spend their money saving time" came to mind with this. Also, I think 20 screws holding this cover on is probably a bit of overkill, since the airward kit comes with like 12 I think. Either way, it's done now and it's gonna work great!
Hopefully tomorrow we'll be able to finish up the rivets on the bottom parts of the skins and be one step closer to putting an end to section 29!