Since I have a camper, Alfred's solution of carrying
full sheets inside doesn't work.  Instead I carry them
on top (no pop top).  You can get inexpensive foam
blocks from a fishing/boating shop made for carrying
canoes.  They are roughly 4x4x6 inches with a groove
in one side. These are VERY handy items.  I use them
in my cupboards for keeping stuff from sliding, as
stands and supports for projects (like cutting plywood)
etc.  Anyway, when I want to carry stuff on top I put
one roughly in each corner with the grooves going 
across.  Then a 2x2 goes across in front and another
in back, each end lying in the groove of a block, and 
sticking out a bit on the outside.  If your plywood is
thick, you're done.  If it flexes and sags, a couple
1x2's down the sides of this thing work fine.  You do
need to be good with knots for tying down the load, and
be careful to pad the rope where it goes over edges that
might cut the rope when it starts vibrating as you go down 
the road.  Be sure to tie down the front end, too, to keep the
whole thing from peeling off backwards.  With care, this
contraption also works, and the possible load can be more
than 4" thick, but you do have to worry about total
WEIGHT so as to not dent the roof.

Steve

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