We use the bunting. I KNOW it hasn't been tested after market, but frankly, it's NEGATIVE 20 and then some around here for eons! The baby has to be warm. Also, I strap the baby in more tightly than anyone I know (without hurting them, of course) - so I'm banking on that being enough to account for any slippage that might occur in an accident - there is NO way that baby is coming out of that seat. Also - I know that some of the brands of buntings in Canada HAVE been tested with no ill effects after market, so I really do believe that they're safe.
There is one that does not interfere with straps. We had pne it was made by Nojo. I will post apicture of what it looked like as the zipper broke I am not sure I kept it...<a href="http://s287.photobucket.com/albums/l...urrent=287.jpg" target="_blank"><img src="http://i287.photobucket.com/albums/l...pmummy/287.jpg" border="0" alt="Photobucket"></a>
Not the best picture
It went over the top of the car seat so it did not come in contact with straps. you just had to be careful not to cover the babies head.
Pardon the red eyes. I still have not edited all of pur photos.