Glad to see the OP got a solution. Just thought I'd throw in my $0.02 for anyone else facing this issue and not wanting to switch coverings for whatever reason.
Monokote can ABSOLUTELY make this curve. I had a nearly identical wingtip on an old Stik design a couple years ago. What I finally figured out with Monokote is that I have to work in very tiny areas at a time for shapes like this, and I must pull the covering quite hard as I work the iron, while simultaneously tacking down the edge AND shrinking the area at and near the edge to make it conform to the edge's shape. I end up with some minor wrinkles in the middle of the covering, but these area easily ironed out. Bottom line: as long as there are no wrinkles on the edges (that is, the areas you can't fix later because they are bonded in-placed to the airframe), the rest will iron out.
Monokote is very "shape-able"...think less about shrinking the covering and more about shaping it. On something like that wingtip, for instance, I will start by tacking down at a point that represents (roughly) the geometric center of the curve. In this case, that would probably be right at the front corner of the wingtip. I then start working the hardest parts (so, working forward), but I don't actually lay the covering completely flat before ironing: I deliberately put some wrinkles in the covering to take up the slack and prevent having a huge pile of extra covering by the time I am finishing the curve. The key is that the amount of excess covering I absorb as I go is not more than what I can smooth out with careful iron-work. This is achieved with gentle pressure as I pull the covering against the airframe, followed by firm iron pressure to tack it down...if you press the slightly-wrinkled covering hard into the airframe right away, you will get a bunch of permanent wrinkles. I am not working against the airframe at first, I am working against the pressure I am holding on the covering with my other hand. This forces the Monokote into a new shape before it is fully tacked down. By the time I get to the end of the job, all that excess covering that the OP was having trouble with initially, is now down to about the same manageable amount I've been absorbing all along.
I used a similar method on a Cub tail, but I would say it was actually harder due to the smaller working area, tighter curve, and concave shape of the final covering. I posted some pics in this thread:
http://www.rcuniverse.com/forum/kit-...-cub-tail.html
I have not used Ultracote since a small repair job I did on a plane when I was about 13, but I am interested in trying it again. My Tiger was originally covered in the stuff (by someone else) and I was impressed at how smooth it looked (maybe just the skill of the builder, I'm not sure). But, at this point I have not been in a hurry to switch from Monokote, since I am so comfortable with it.
I also wanted to mention that getting FRESH Monokote is CRITICAL. Recently, I had some really old stuff I got from a friend, and it was a nightmare. Dumping it and going to one of my new rolls made all the difference in the world. I am also aware of reports that Monokote's quality has been hit-and-miss in recent years, which is really unfortunate. Perhaps I've been lucky, or perhaps it's just the colors I use (typically red, white, blue, or silver), but I have had no such issues as of yet.
Matt