You can also make spacers from blue board sold by Gaylord. Look in the catalog and they have pre-scored spacers, but you can certainly make your own after seeing the example. You can put a sheet or two of buffered paper between the blue board and the objects, if needed.
You wrote “box” as in “one” — many brands of photocopier paper are acid free. Can use them as filler.
I have blue board and acid free tissue on hand and use them frequently as we continue cataloging a huge back log. 6000 done and probably 1500 to go.
On another note, I find having and using an acid-tester pen of value. It is quick and lets you know acid or not.