I finally put the strap holes into the carseat cover. I had to do it completely by hand as my machine has a special buttonhole foot which doesnt extend big enough for the strap holes (who said newer was better?). So, dont look too closely!
I made it with fleece one side, pink cord on the other and a layer of batting in the middle so it had some padding. I traced the contours of the seat and made my own pattern then just sewed it together, turned inside out and hand sewed the hole up.