Carseat manufacturers and the AAP strongly discourage using aftermarket products of any kind and consider them unsafe. In fact using aftermarket products will likely void the warranty of your CRS.
If you search your CRS manual you will likely find something like this:
From the Britax manual:
"The use of non-Britax Child Safety, Inc covers, inserts, toys, accessories, or tightening devices is not approved by Britax. Their use could cause this restraint to fail Federal Safety Standards or perform worse in a crash. Their use automatically voids the Britax warranty." (This statement can be found under the Warranty section of all Britax Child Restraint manuals)"
Here is a quote from the AAP website:
Are you using an unsafe product on or with your car seat? You might be if you are using an aftermarket product.
An aftermarket product is anything that is not sold with your car seat. Shoulder belt positioning devices, seat belt tightening devices, head/foot cushions, travel tray and car seat covers are considered unsafe aftermarket products. It is never recommended that you use anything extra with your car seat without the express consent of the car seat manufacturer.
Here is the link for anyone interested,
http://www.cpsafety.com/articles/AftMarketMan.aspx
Great resource for carseat saftey information.
HTH
TJ