Someone who lives not far from me has an FP1 in their basement which apparently was delivered without a horizontal table. Buying one was too expensive, so he made one from aluminium tooling plate that has a grid. Half the holes are precision bored for dowel pins and the other half are threaded. Seems to work well for him.
On my FP2, the main limitation on size of work is the travel, not the size of the table. Having said that, a while ago I did some work to the table from my Studer cylindrical grinder, which was a bit too long to fit on the FP2:
Final clean-up was with the horizontal spindle, after measuring and mapping the deviation from parallel on the grinder:
This was to mount a Renishaw scale under the table, final result is here:
The gold-colored scale strip is 600mm long, and the underlying steel strip is about 120mm longer. This was a challenge, since the X travel of the FP2 is just 500mm. In the end the strip measured deviation from parallel to the travel of the grinder was 0.03mm (0.0012"). Renishaw specs allow up to 0.1mm deviation.