Don't go less than 5 mm for the top thickness (copper top)! You could have some troubles fitting the barbs in it and leaks...
I never had any troubles fitting the barbs in 10 mm top (but it was bulky and heavy), and 5 mm thick top is IMHO a minimum, and I used a fair amount of teflon tape to seal the barbs in it. Could be better to use some liquid sealing (non permanent) from Loctite or similar.
Good luck!
