fostuþr is just conservative West Norse for inscribed fastaþr. Some
of these names could originally have been in fost-, as ON names in -
fastr are u-stem (gen. -fastar) showing older -fostr in the
nominitive, but the change *fastu > fost > fast- (in names) is
probably older than ON in these compounds. Ok, enough changes.
Use the saga-literature and we add:
fastný
Let us praise the Uppsala-Swedes for inscribing these beautiful ON
names and saving them for us, while their neighbors had forgotten
them, or were importing non-ON ones while they probably should just
have saved this category instead. Add the other sources, and we get:
OS fastvidher, etc. solid. fastráþr and fastfrøþr are exceptions
here, being preserved in Old English, but about Norse persons in
England. No new constructions, so our total: