I have seen people just use a thicker piece of MDF board and then cut some MDF board to act as the sides as well.
Here is what I do: I head down to the local Home Depot/Lowes. I pick up sheet metal flashing. They sell one sheet that is flat (about $6) and one sheet that has a 2 inch(ish) lip on the back (about the same price as the flat one). Once I get that home, I measure out my projected tray (with no borders). The way that I do this is to use my magnatized models ranked up to fill the tray. I get the tinsnips and cut my tray out. Then I sand down the edges (this can be dangerous, wear gloves). Once I have smooth edges, I paint the tray green. Once it dries, it's done. Here is one of my trays with some zombified skrox on it: Here is another one made for a crapton of skinks:
Oh wow thats a good idea... i woulda never thought of that. And the magnetic strips for the bottoms of the models?
From staples or office depot etc sell rolls of magnetic strips for 5 bucks for a few feet worth. Its a roll like masking tape. One side is peel off sticky, just gotta cut it ever inch.
I used that metal flashing for the movement bases and then used the edges from the GW movement trays. I tried to use the new GW glue to attach the two and it would really stay, half my trays came apart day after the old glue seemed to work fine.
That's a good idea too N810. Michaels (or any good arts and crafts store) also sells sheets of square magnets that are an exact match to the 20mm (skink size) bases. I also use rare earth magnets too.
Craft stores sell the 20mm magnet size! Well damnit. I spent like 40 bucks ordering from hasslefreeminatures cause I was too lazy to cut the magnetic tape anymore.