if WOOD is what you love do it in WOOD
she'll be your baby not someone elses
the look of WOOD is WARM & FRIENDLY
WESTERN RED CEDAR fencing material comes in 4" & 6" widths and is typically 5/8" thick w/ one smooth side & one rough side
WRC is light in physical weight, quite rot resistant, takes stain beautifully, then varnished for a natural wood finished product
many racing sailboats up to and thru the late 1960's were planked w/ WRC and finished natural
there is almost nothing any more beautiful than a yacht finished sailboat that is planked w/ WRC
if you have access to a thickness planer & a table saw or router, you can thin the planks down to 1/2" and cut the edges similar to LAP SIDING
over a 1/4" plywood(BCX-YP @ under $20/sheet) outer skin you can then add the WRC to achieve your desired look, whether in a horizontal or diagonal pattern
a combination of PL-PREMIUM adhesive and some carefully placed metalic fasteners(RINGSHANKED BRONZE BOAT NAILS would look KILLER, IMHO) for applicatiln of the WRC should do the trick
i agree w/ the suggestion to use thicker framing, which can be ripped out of virtually any species of "1 by" materials, to accomodate 3/4" foam insulating layer and ultimately skin the interior w/ 1/8" door skin ply(very common and inexpensive) for a smooth attractive finishable surface that you can "paper" w/ maps to your heart'z content...
the above suggestion to use EPOXY as a coating on the outside is not wrong, the folks who suggested it just forgot to tell you that most EPOXIES DON'T HAVE UV INHIBITORS in them and will eventually dull and go away, therefore EPOXY must be coated w/ a UV inhibiting coating like varnish...
1/4" marine plywood of miranti or okoume would, either one, look great finished w/ stain a/o varnish and would run you between $50 & $100/sheet depending on where you get it, if you pick it up or if you have to have it shipped to you
IF YOU LIKE/WANT WOOD BUILD IT W/ WOOD
sw