The only bottom metal that will work with the FN is their TBM with their proprietary mags. AI mags require some very minor customization of the action.
Not sure if the new PTG DBM for FN has some kind of work around, but if it uses AI mags I doubt it.
CDI can do the milling, it's a very tiny amount of material at the from of the receiver opening, that's all. Any smith that's familiar with FNs can do it in 5 minutes.
In addition to having CDI do it, as an alternative I recommend Accurate Ordnance for all things Win/FN, their work is impeccable.
PM Bookhound (Mark at AO) if you really want the technical explanation.
Seriously, it's not a big deal. If you have a local smith you trust he could even do it after a quick call to CDI or a little Internet research on how the cuts are placed.