Same i build my own too. I used coated chipboard and fiberboard for the first one and im using marine plywood for the second. I will let you know what works best when i have finished.
on about the depth so once fitted you can put the glass doors in. hard to explain but you put top in first then it fill go over the bottom one in drop in.
I understand Daz, that's what I thought, you slot it in the top runner and then drop it down into the bottom one, but you have to explain these things to Matty ;)