what is the best way to repair dacron?

I recommend you just make an inspection panel/hole type patch there,

To install a standard fabric inspection hole on the bottom side of the wing, first locate the center of the hole to be cut. Clean and sand the surface affected down to the primer. Using compatible fabric glue, glue a 1-617 reinforcement ring into place followed by a 1-268-1 fabric patch over the top. Allow the glue to dry . Lightly iron the area, Cover with 3-639 cover.

You can overlap them to cover your 8" long slit.

Even though your Himax is an ultralight, you still need to repair it using standard aircraft repair techniques. If you want to do it right, you should consult the manufacturer of that material. If you don't know, and there are no builders logs specifying the fabric and finish process that was used, it is a fair guess to say that it is Stits Poly Fiber, the main manufacturer and supplier of dacron aircraft covering in the US. I suggest you contact them, ask some questions, and order their book / video on fabric covering and repairs and follow their recommendations. See http://www.polyfiber.com/stits/index.htm

what are you talking about? I've never heard of a dacron and i'm a student pilot.