I needed something that will make it easier to replace couplers, apply decals, or do other work on a car or locomotive, while minimizing the amount of damage done to the other side of the model. While one can purchase commercial foam-based cradles, I decided to make one from scratch from materials I had in my garage. Measurements don't really matter, just so long as it is big enough to provide enough stability for the models you are going to place on it. I made mine out of two pieces of 3/4" plywood that measure 3.5" wide and 13.5" long. I simply glued the long end of one of those pieces to the top face of the other piece, making for a 90-degree joint. After that dried, I placed two pieces of 1" high-density foam (I bought a roll of this many years ago at a local Joann Fabric and Craft store). I originally used it to pad boxes I used to transport my models to local train shows (hence the little bit of dirt on the foam), but since I don't do that anymore, I have these leftover.
(external link: Foam at Joann)
The foam pieces measure 4.25" by 10". I marked off on the wood which areas would be covered by the foam, then applied a thin coat of wood glue to that marked-off area, and placed the foam in place. I put a couple of small metal weights on the foam to make sure that it remained in contact with the glue while it dried. This then left a small area of wood exposed on either side of the foam. I then had the idea of gluing a thin piece of plywood leftover from a sprue of a laser-cut wood kit and glued them on either side. I now have a handy built-in catch-all pocket on either side. This came in handy when I replaced couplers, as the screws naturally fell off of the model and right into those pockets, without rolling off of the workbench.
I wanted my cradle to sit at a 45-degree angle to make it easier to work on sides of a model. So, I cut two pieces of 1/4" plywood long enough to provide enough stability to the whole cradle. I then marked off the cut-out I wanted to make in the legs, to fit the 90-degree cradle. I clamped both pieces together and then cut along the marked lines. After a bit of final filing and sanding, I glued them to the bottom of the cradle, making sure that the legs where flat on a flat surface, and vertically straight.