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