Water Expansion Tank

I'm thinking of having a custom expansion tank made up to replicate the older style MK1 units but will fit into an MDA. My MDA in particular.
What points do I need to look out for ?
I have a few initial thoughts of my own

1. What pressure Cap should I look to buy. 19-20psi, 22-24psi or 29-31psi ? What determines this requirement.

2. What guage of aluminium should I fabricate the tank in.

Any help would be appreciated.

Martin, minimum 2mm thick with ribs in any flat surface greater than 100mm wide. Make sure the welding is perfect because I have seen a lot of tanks split at seems. TRY TO RESTRICT THE SYSTEM to 15 PSI if you have good cooling. You have some of our ali work ! Frank