Making your spray skirt tunnel smaller - or larger...
The first spray skirt I owned was too big for me (the yard sale special). Using sharp scissors, I made a clean, V-shaped cut in the back of the tunnel, over most of its length. Then I used neoprene cement (the black goo which you allow to touch dry before mating the surfaces) to glue the cut edges together. Then I sealed the seam with aquaseal (the stuff that dries clear). It worked fine until I got a new boat with a different sized cockpit and needed a new skirt anyway. My current skirt is now getting a bit old and slack, so I think I may repeat the process with it.
You can also insert a piece of neoprene the same way to make the tunnel larger, so save the piece you cut out, in case the tripple whoppers get the better of you.
Good luck, stay dry.