I am not a fan of metal trays... I had a friend who used them, and when I once helped him put his army away, I ended up cutting my finger pretty severely...
I use sheet styrene. Its easy to cut, glue and you can lay either a metal liner into it or use sheet steel (its like sheet magnets material but not magnetized itself).
I use 2mm sheet Styrene with 2mm x 4mm styrene Rods.
Here is how I make them.
http://allyourswords.blogspot.com/2010/08/ive-got-movement.html