I've been very happy with black self adhesive 3/16" gatorboard. The key for me has been to have a mounting pressure roller system called a Polar Manual
Roll Laminator. This applies even pressure to mount the print securely to the board.
I found this to be very easy to do. I do leave space to trim the print to the desired size. For example, I mount 12x18 prints on 13x19 gatorboard and then trim with a razor knife and a steel straight edge. Works great.
I have no idea about how long this will last, but I don't really care as I switch out the prints every 6 months or so.
M. Van Drunen