As for rainbow tables regarding the 16 lenght number default password on FritzBox , YES, if somebody had them already made, you could calculate for your FritzBox if it has a default password or no (all possibilities, 16 numbers).
But as regarding to MAKE a rainbow-table for 16lenght numbers for FritzBox, it would take 63 years (same time as dictionary attack or even a bit slower) and some PB , thousnds and thausends of TB of space. But yes, if anyone would make a rainbowtable with some super-computer and store it on his super-drive, it would work for all future probes (ofcourse only with same ESSID (wlan NAME) , for example only for "Fritz!Box FON Wlan 7390". For EVERY ESSID you would have to make a new rainbow table.
In practice, I higlhly doubt that anyone has a rainbow table for any of Fritz!Box which include all default WPA's (16 lenght numeric key).
If I were you, I would try to calculate the algorithm, with which they (the factory) calculate the default WPA key. You can find at least a dozen of FORUMs, WIKIs etc (most in German) ,where they are trying to calulate the secret, from which the factory calculates the default WPA keys for their boxes. As I understood , the calculate them from the MAC (every box has more than 1 MAC, one for WLAN, one for ETH, itd. , from serial number, and from default ESSID.
Good luck decrypting the alghoritm.