Walmart offers the Stalwart Electronic Extra Large Safe in Black for $69.99 with free shipping. That's $43 off list and the lowest price we could find, although we saw it for $5 less last May. It measures about 20" x 12" x 14" and features a digital lock with two manual override keys and a LED display. Four AA batteries are included.
Uncle Sam could take a hefty chunk of your retirement income if you botch this.