This is the cartridge guide needed to adapt the Flashback 2 to use the original cartridges. The Atari ones have an integral dust cover that interfaces with the bosses and the slots to expose the card edge. It's designed for use with this card edge connector:

http://www.digikey.com/product-search/en?WT.z_header=search_go&lang=en&site=us&keywords=S3304-ND&formaction=on

And you can read all about the Flashback 2 mod here:

http://www.atarimuseum.com/fb2hacks/

flashback 2 mod instructions here

http://www.atarimuseum.com/fb2hacks/

The cartridge guide might need the holes drilled out to match your screws. The card edge connector slot is a tight fit on purpose. It should stay in place just by itself, but there's a little tab on the bottom to hold it in place just in case. You can mount it wherever you want, I just screwed it to the top.