If you are in the US, just buy the Falcon Memory Upgrade board from Wizztronics. It accepts a 16MB 72-pin SIMM and is the same size as the Atari memory cards. http://www.wizztronics.com
Best Electronics sells it too.
If you can find the WB Systemtecnik one, that one is suppose to be really good.
Avoid the RAM Gizmo. It's bulky and may be tough to fit without removing something like the speaker. It also has problems with accelerators.