I have created a cheap, somewhat ugly but very workable duct to bring cool
air from the rear A/C ducts right up to the front.
Start with a length of deflect-o brand semi-rigid flexible aluminum duct. It
comes in 3" diam. by 8' length which is only about 3' when you buy it but
expands to 8'. It was less than $6 at Ace. It is very lightweight and
retains the shape into which you bend it.
Start by stretching out about 8" of one end as much as possible and then
carefully flattening it into a rectangular shape about the shape of a 2x4 or
a little wider. The end is going to fit into one of the vent openings and I
used the far right one, so carefully pry out the vent grille. Use a
screwdriver to push in on the tab on one end. The opening is 1 1/4" high, so
that determines the shape of the end of the duct. Flatten the end of the
duct completely with duck-bill pliers and snip the corners about 3/8" back.
Fold each side back to form a hem to strengthen it. Shape the end so it will
fit into the opening about  half an inch. When you've gotten that done, just
start extending and bending the rest of the duct according to the route you
want to use to take it to the front. Mine runs up above the air vents above
the sliding door and then curves down diagonally across the back of the
passenger seat head restraint, so the open end is facing the drivers area.
It is so light, it doesn't require much to hold it in place. I used some
light cord. I have the upper bunk removed and small hooks along the leading
edge, so the duct is tied to one of those just above the vent it is coming
out of. My fire extinguisher is mounted up above on the flat ledge above the
door, so the duct is tied again to it's bracket. It's also tied to the base
of the pop-top support and lastly, the back of the head restraint. I used
some thin soft foam to help hold it where it is tied. Where the end enters
the duct, push it over to one side and seal the rest of the opening with a
square of soft foam.
That's all there is to it. You could make a second one for the passenger
seat, just doing everything on the other side.
It looks kind of industrial, but could be covered with some kind of stretchy
material. Best of all though, it really works. I can turn down the fan speed
and the temp. It was cheap, fast and is removable at the end of the season.

Jerry