Very nice work guys!

I like to use polystyrene foam sheets (trimmed to EXACTLY 1 3/8" thick) then use the 3D CAD model to create 'slices' every 1 1/2". I plot the section of the slices, 3M77 them to 1/8" lite ply then cut to size. I use copper pipe or conduit for alignment (usually 2 pipes minimum so I can keep everything straight) much like JohnW did. I cut the foam sheets slightly oversize (at the 'large' end) then I sandwich the lite ply and foam sheets together through the pipes using 3M77 to hold everything together. I throw this into the vacuum bag for a few minutes so that everything sticks together well. I sand the foam down until I hit lite ply, then fill any mistakes, sand, then glass with 3 layers of 3.2 oz crowsfoot cloth. After curing it's prime, final sand, polish, mold release, DONE! I wish it went as fast as it sounds!

IMO this method is pretty fast and you end up with a rigid (well, rigid enough) plug.

-Tom