Jack,
Is this the profile P-40, Pat's design?
the amount of dihedral will really not cause a problem with the flap horn. The thing to be aware of is that you want the horn to make a straight line from the flap hole where the rod goes in, across the trailing edge, to the other flap hole. This means when you put your horn busing into the trailing edge, it will not be centered in the trailing edge. it will actually need to be a little bit high on the trailing edge.
for this profile, you could just take some sheet butyrate plastic and after making a frame, ( actually doesnt the kit have a frame, kinda thick but still,,,,) then basically laminate the flat sheet to each side of the "frame"
I am kind of surprosed there are no horns included,, I am sure we could come up with some,, though I would have thought they would be included, perhaps it was an oversite on the kit, maybe a call to Brodaks to clarify is in order.
If you are talking about making the fuse basically empty, with some "ribs:" then sheeting it, YES its the way to go, lots lighter,, LOTS
Jack, if ya wanna talk through this, give me a call, you can private message me through here, or I am in the Pampa guide as well.

Jack,

Listen to Mark. He's built a lot of these. A lot.

well I certainly have had oppurtunity to build a few of them,, and I do have a soft spot for them,,
I dont want to blame the airplane for the way I fly, but they HAVE allowed me a lot of progress, lol

OUCH,,
the sharpest knife cuts the deepest,, yeah,, that one didnt give me a lot of practical experience with my flying,, damn dandelions,,

that is interesting Bill,
I have a wing sitting on my chifrobe right now, plus a set of ribs besides.
Right now I have two projects to finish , my new profile electric, and my long ignored bf 109 that is ready for paint,,,
then of course I need a backup Pylon race plane before I take on any new projects,, but it certainly is interesting,,

Steve,
I am flying 424, otherwise known as APRA, its the "slow" version of quickie 500.
mostly Vipers, stock engine class, thunder tiger Pro .40 , stock prop. According to scuttlebut and those who report to know, they do about 130  mph. closed course with a few other like minded pilots. It is a total addreneline rush. Todd got me started, now he is flying the next step up, composite airframes and Nelson engines,, a bit pricey for me right now...