I have made numerous sheets from the Suitability pattern. I assume that is the pattern you purchased? For my daughter I made one with coated cordura on the outside and wool on the inside. I suggest that you preshrink the fabric before sewing it. It will get wet when the horse is wearing it no matter how much you waterproof it and then the lining may shrink more than the outer shell and cause bubbling or twisting.
Another tip--I used 1" elastic for the leg straps for give and 2" elastic for the belly band. I had seen that on purchased turnouts and incorporated it!
It's a nice pattern, if a little daunting because you have to draw it yourself. I just got a purchased blanket that I knew fit and traced around that, allowing for the darts in the rump.
By the way, burgundy is my favorite color! I'm sure your blanket will turn out lovely.