I had this talk with Van himself at Sun-n-Fun last year and he said that what we were doing wrong was that we were not trimming the wing root wing skins so that they had at least 1/8 " clearance all the way around. I would have done this but the aircraft was already painted and I did not want to make a mess of the wing root. It has still not made it into the plans so I guess we must just read Ken's mind on the subject.
The material I used was from Home Depot, it is the MD brand, High Density Foam Tape Weatherstrip, the size is 3/16" thick x 3/8" wide x 17' long. I used the white color and it comes with two rolls in the package which is plenty to do both sides of the plane. The aircraft is hangared and I see no indication of any change in the material after six months so it would seem to hold up well and at about $ 2 per package it is cheap to replace.
I start with the wings on and run a 1/8" masking tape line around the wing root even with the wing skin on the fuselage. I then remove the wings and place a second 1/8" tape line outside and against the first tape line. Then remove the first masking tape line and place the foam tape to match the inside of the second masking tape line. Remove the second masking tape line and replace the wings and let sit for 24 hours or so so that the foam tape will have some clamping pressure to keep it in place while the tape bond develops.
I might add that my wings fit with only 1/8" or less gap between the wing and the fuselage if your gap is wider by much you may need thicker foam tape and it is available.
Good luck, hope this helps.
Best regards,
Vern