Any suggestions for an antenna similar to the METZ which
can mount atop a 4-foot extension and screw into the fitting
on the extension?

I suppose it's still available, and I confess I didn't like it as well as
the Metz, but a Shakespeare Style 5240-R VHF 3dB fits pretty much the scheme
you are looking at. They seemed to work well enough, once in a while we
found a "dud", so be sure and check the SWR after it's installed to be sure
you didn't get a defective one. Never saw one go bad after installation
unless it got physically "smashed".
Old Chief Lynn