I chose the Sidewalk sheet because it comes pre-scored with grids. By scoring lightly along these gridlines, I could snap the pieces off and end up with a base with perfect straight lines and right angles. The rectangular cross-sectioned strips were chosen over the square ones because I wanted my trays to have a narrow and tall lip. Using a pair of clippers, it was easy to cut these at right angles.
As the material is styrene,they can be glued together using the same Tamiya glue that I used for the figures. I glued the strips to the reverse side of the base, which is smooth. And there you have it.
A coat of primer and paint and some flocking on the lips and we're ready.
The sheet costs about US$8 and the strips about US$5, and will give two 5 x 4 infantry trays and two 5 x 1 cavalry trays. I'll probably need another sheet for the whole army, but the numbers seem good.