The nice thing about this is that it is (almost) free, and you can make it a custom fit to ensure that your dog will be snug as a bug in his/her coat. The only thing I ended up buying from the store for this project was a few strips of sew-on velcro (about $7.49 per yard for 2" wide strips and I only used about 1 foot).
IMPORTANT NOTE: I HAVE BARE MINIMUM SKILLS USING A SEWING MACHINE, THIS IS NOT A DIFFICULT PROJECT!
Old fleece coat or vest
Sew-on velcro (2" wide strips)
X-acto knife (or seam ripper)
Step 1: Prepping the vest
The length of the coat should be measured from just below their head and down their back to the tail (for me, this was about 30" and was a pretty perfect match for the XL vest).
The first step was to remove the main zipper. I cut the seams on either side with an X-acto knife (you could use a seam-ripper but I found the North Face seams to be incredibly tight so the knife was easier). I did this carefully so that I could use the zipper in the future if I wanted to.
My goal with this project was to use as many of the existing seams as possible to cut down on having to sew hems (due to the thickness of the material).
The vest was assembled in four primary pieces (large back panel, and two front panels that came about two-thirds of the way up to meet a shoulder panel). I then cut the two front panels off at their seams (down the sides under the arm pits) leaving the large back panel and shoulder panel.
I wanted to use the large back panel as the primary part of the coat down Klaus's back and use the shoulder panel with the collar to go over his head and cover the front of his chest.