When living in CA, I built a walk in that was about 100 sq ft. bilt of 8 inch concrete block with the cores filled, on top it had a 4 inch slab of concrete with re bar 4 inches on center all protected by a big ole Diebold vault door that I bought from a used safe company. If you could get in you could have what was in there.
Get bigger than you think you will ever need...lots bigger
Goldenrods...
I also like the ones that use quick drying concrete between the layers of steel. Not only is it a deterrent it acts as fire protection.
I also would check the used safe companies for what they have on hand as in used bank vault type of stuff.(big,tall & wide) Some of it is so heavy that you need a Rigger to bring it in. as in a ton or more
It is easy to build interiors..
Spend the dinero and do it rite, you will only do it once. If U move well then you call out the guys to move it.