Absolutely no need to secure. I have a lead ingot (about 40 lbs) just sitting for no reason whatsoever on my house battery tray (aft of the batteries). It has sat there for 5+ years without moving even a fraction of an inch. I have been out in small-craft advisory conditions, been waked so hard everything in the boat was on the deck, etc etc. All of my lead ballast is unsecured, and has never moved.
If you do this though, make sure you can still get access later if needed, and do not use foam.
Just my 2-cents.