As far as the gap... I took medium density foam and glued it full length to the lift gate. When you close the tail gate, it compresses the foam slightly making the seal. A simple rectangular piece of foam works great for dust, but if you want to seal it from rain, you'll have to use a much thicker piece of foam (~1.5 in or so). It will need to be cut so that the foam provides a roof over the tailgate, but thin where the tailgate closes. Cross-section below....

________
| |
| (foam) | --(lift gate side - clamshell)
|____ | (glue this edge to lift gate)
-----| |
| || |
/\ | ||___|
||=| |
\/ | |
| |
| | --(tailgate - closed position)
| |
| |