From my building experience, the most bang for the
buck is PVC pipe. Parts are cheap. You don't have to have
a shop and they look like they're from the future.

Just about any full range driver will work-- RS 1197
or upscale FE127e's. There's prob'ly a 4 " from Tangband
that will work too.

You mount the speak on one end and the other end is open.
Allow about 80 inches in between. Supply elbows to shape
to taste.

Here's a shorter version:

• PVC periscopes