I just got a probe and needed to make a sled. The manual suggests a laminated surface so that you can hot glue things down and clean it up, which made sense. I rememberd that I had a secton of counter top from a sink cut-out left by the previous owner of my house. I've seen these cut out pieces for sale at a local home center for $3-$4 each as well.
All the pieces came out of the sink cut-out. I used a router to remove the laminate from the edges of the base to leave a glue-able surface for the sides. The sides and base were joined with biscuts and glue. the side I wound up with about a 11.5" x 30" scanning surface with 2" sides.