This was an issue that has been bothering me for the past two years. Every time I looked at my wing tips was just grrrrrr! I made the aluminium wing tips from the dimensions shown on the plans, however my were slightly undersized which resulted in the upper surface of the wing skin rippling and just looking a little ugly. I had initially experimented with placing packing pieces under the the rivets between the skin and the rib, but it just looked like a bodge up. I plucked up the courage a couple of weeks ago, made another form block slightly over size by 5/16" down to 1/8" along the upper surface. I initially tried just drilling the old rivets out, but you only need one to start spinning and you're stuck. Plus the stainless rivets are hard work to drill out at the best of times. I ended up taking a deep breath and used a nibbler to cut out the end panel and simply ground the back of the rivets off with an angle grinder. It still took me a day's work to make one end plate, plus a couple of new angle brackets. It was far easier to make the new mounting brackets than to try and line up the holes.
It takes a solid couple of hours just to get to this point. You can see the new form block plus the blank cut out and ready for some metal bashing.
In this picture I am working my way along the top edge folding it over to the 135 degree angle. I have learned to cheat a little, the edge that is folded right over I allow 3/4" flange instead of the 1/2" which makes it a bit easier to line up when drilling the holes.