Don't use bondo, what is the kit made of? Fiberglass or Polyurethane?
If its fiberglass, just get some glass matt and some resin. Repair it from the backside. Then get some fiberglass called kittyhair. Its chop matt mixed with a fiberglass paste thats very easy to form and sand.
If its polyurethane, an autobody supply shop will carry an industrail plastic glue that will seal the seam. Again do it from the backside then use the kittyhair on the outside.
Make sure you prep the area really well so everything bonds together good.