Hi Yves,
I think which method you choose depends on what you are trying to achieve:
1. If you want a solid plate, then I would create a back face. The problem with this is that this will create a print with a large volume which will be quite (very!) expensive. Perhaps you already know how to do this, but the method I would use is:
- Select Edit Mode;
- Select all the Vertices at the back of the plate;
- Press E for extrude and then immediately press Esc;
- Merge the Vertices at Center.
2. If the plate does not need to be solid, but you just want to give it a certain thickness, then I would use the Solidify Modifier. I see that you have already set up the units, so you can directly enter whatever thickness you want. The only problem here is that with such a large print maybe it will warp when it prints (i,e, the print will be slightly curved) - so it might be an idea to add some internal bracing structures.
Good luck