Designing fuses is harder than it sounds. Lets start with a very rough approximation, the fusing current for 15 gauge copper wire is normally given as 140A. The cross section of that wire is 1.7mm^2, so that is about the cross section you need. If you are using 0.1mm thick foil, then you'll need it to be 17mm wide. Since that is wider than the 10mm ends, you'll probably want to go up to (e.g.) 0.4mm thick foil, and 4mm wide.
But that could fuse anywhere over quite a wide current range depending on:
- heat flow from the fuse to the air, which depends on:
- air temperature (only if it's hundreds of degrees - but it could be next to the high current wires)
- orientation of the fuse, flat, vertical, on end...
- humidity and air pressure
- air flow, enhanced by a fan, or restricted by a case
- heat flow to the bus bars, which depends on:
- the size of the tabs at the ends
- how the tabs are secured to the bus bar
- the material and size of the bus bar
- the temperature of the bus bar with high currents flowing through it
- The copper alloy actually used, resistance and melting point
- The details of the overload, such as how long it's been at 70A heating up.
- Any damage to the (soft) copper fuse during installation.
In addition to fusing current you should also consider:
- Arc extinguishing. The copper may establish an arc as it melts, which may continue to conduct until extinguished by a voltage reduction, strong air blast etc.
- Where the molten copper goes. It will be expelled with some force. You may need a tough case around the fuse to stop it destroying something or killing someone.
- Whether running for long periods at 70A will heat the copper enough to soften it, and let it bend or deform, increasing resistance.
You won't be able to calculate all these. So if you are set on making your own fuses, you need to commit to a series of tests. Start by getting a dozen fuses with cross sections between about 1.2mm^2 and 2.2mm^2, and set them up in a mock-up of your equipment with the right temperature, airflow etc. See where they fuse, pick the best size, and get more made. Test these in all the variations of conditions you expect to see in the field. Exactly how much testing you do will depend on what level of protection you need.
Given that 150A bolt down automotive fuses are not very expensive this kind of testing is probably only worth it if you plan on making lots of these.