I just made a set of copper gaskets for my t78 using 3mm copper plate,this seemed like a good thickness to withstand the heat.Anyhow if you're going to do this bear in mind that the copper doesn't have a crush ring like your conventional gaskets.
This means you need to make sure your mating faces are very true,otherwise even a .001" gap will blow like a bitch.
A thinner gauge plate may allow a litltle more crush,but personally i didn't want to go any thinner.What you'll be needing to look at is the hardness of the material you use,you'll want something pretty soft that will crush enough to make allowances for the distotion of the mating faces.
I used a medium hardness,don't know what it is in hardness measures but i was told it was medium hard plate.You can anneal the plate a little using a plumbers torch etc. to soften it if you find it doesn't crush enough,looks like i'll be doing that anyway,got a .001" gap to loose!