Phil Basset wants to dovetail long boards but doesn't have a hole in
the floor to drop the long end through (Old timers will recognize the
story reference).

Phil - if either you or a friendly neighbor have a raised deck plus a
couple of clamps, your problems are over. Position the end of the
board to be cut at the appropriate height for yourself by fastening it
with clamps to the outside of his/her vertical deck rail. Then stand
on the deck and cut away.

FWIW

Alan N. Graham