Ok thanks for these. In the end I have closed them without I stitching everything.. phew. I put the boat on a pair of sawhorses without the cradles so that the whole weight bore down on the keel at the point where it touched the horses. The gap closed and I was able to pull the zip ties nearest to the horse tighter. Moving along and adding the weight of a small toolbox to the bow bulkhead I was able to get them all tight.
You might fine putting ties in between the ones you have will help pull the bottom together.
Sometimes wire ties work better than plastic zip ties. you can twist them tighter.