You have to trim the vacuform fairings to fit the vacuform fins. The spot I believe you are refering to is where I over-trimmed the fairing.
I plan on using a little putty to fill that void. You can have a look the image I posted a week or so ago and you can see what the parts look like before they are trimmed.
I remember you, I hope your Saturn 1b turned out ok. Yes, I still get orders for S1b wraps every so often. I trying to concretrate on The LJ parts right now. I will let you know when the LJ parts become available.