I would like to use a light weight half-wave antenna,
possibly something similar to a METZ which has a
34-inch whip and matching coil base, and mount this
atop a 4-foot extension on my 22-foot power boat.

The problem with the METZ antenna is that it does not
look like it will mount on the usual fiberglass extension
mast, unless you use some sort of C-shaped adapter.

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?

My interest in this arrangement is to permit the base of the
antenna to be elevated at least four feet above the mount.
Using a typical 8-foot antenna puts most of my signal
right into my helm station, due to the location of the
antenna mounting base. I'd rather not move the base, just
elevate the antenna.

The 4-foot extension plus 4-foot fiberglass antenna combination
will probably be too heavy for the mount to handle, and that is
why I am looking for something like the Metz--less weight aloft.