To cut the corners I started by making a template off the old tailgate, make sure one edge is long so to have a good straight reference, mine was about 300x150, 300 on the bottom.
Trace the curve of the og tailgate corner and cut it as close to exact as possible, most of the factory boots don't actually have smooth corners, but rather 3 or 4 "flats" along the "curve".
Lay that on the inside of the adventras boot, flat edges on flat edges , the trace the curve.
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DO NOT CUT THIS LINE
You will need to mark the 1st cut line 10mm wider(closer to the corners) so you have the metal free to form and fold it around.
Next bit is to unpick the tailgate seams at the corners, even tho you only need about 100mm from either side, you'll need to start the flattening of the seams about 155 up so you don't over stress the fold.
I used a razor to clean the seam sealer off, the a small cold chisel ground to an almost sharp point so it would fit between the layers but not mangle the outside layer.
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Pic halfway through flattening seam (my lines are also on the other side so my wide cut line is there aswell)
You need to fold the seam out almost 180 so it's flat with the outer skin just past the "cutting line" the rest of the bit you unpicked is to avoid over stressing the metal, not really needed but it saves extra metalwork later to fix it.
Now comes the part you don't want to mess up, you have 2 cut lines well call the 1 and 2 to save me messing up what I'm calling them. Line 1 is the one closest to the corner, line 2 is the inside one.
Line 1 gets through both layers that forms the curve we need, with excess to fold over.
line 2 ONLY gets cut through the steel for the inner skin (the line in the silver tailgate photo)
Now you should be left with a small curve on the inner skin that roughly follows the actuall boots lines and a curve roughly 10mm bigger on the outer (should line up with the outside of the edges of the flattened seam.)
Now comes the "fun" part,
Forming the seam.
I cut small pie cuts out roughly every 15mm (from memory I think I just broke the radius up a few times evenly maybe 6 or 8 times)
<Edit added photos of template up top>
Make sure these cuts DO NOT touch the inner skins curve as you don't want them visible from the front or side when it's done.
Now just fold them around, start at one end and do a couple of taps on each tab then down the seam and rinse and repeate, do it slowly untill the tabs are 90° (pointing towards you(same position as my photo))
This is the last time you'll get a good chance to see the curve that you need to form it to, give the tabs a few love taps to shrink the gaps and go back to flattening the seam. Once it's done you're bound to have some little imperfections so hammer them out.
Now it's time for the other corner xD
If you feel I missed anything or just did a **** job explaining it let me know this will go towards a 1st draft of my actual write up