By George it fits!

His name is actually Ollie.  Had to show a picture of the swatched dog sweater-  I'm surprised and glad it fits .  Cute, eh?

Below are some pictures of the construction in case you want to make one.

Of course you'll need to do a swatch for the gusset to get it the size you need. Just because I am sometimes lazy, I used a 2 x 1 mock rib. If knitting a dog sweater for a larger dog, the ordinary swatch probably isn't going to be big enough.  If you can, measure the length and width you want and knit your "swatch" / rectangle that size. Then use the gusset size to fit.