I've been pushing the NIMHs over Lipos for tanks for a while and I totally agree with not NEEDING them in the tanks, but at some point we will have to swap over to a new battery. They are starting to get in the 20lb range soon for the modern tanks, bigger motors, and more features so it only makes sense in the future. Many things have changed in lipo tech over the years. If you haven't looked into LIFE batteries then check those out, they are a ton safer than even li-ion batteries. While the discharge isn't as high as other lithium packs that is ok for our application since the current demand is pretty low.
Also the old ammo can method is a bit outdated at this time. There is a product called the Bat-Safe which is amazing and works pretty darn good. There is a video on YT of it containing x15 4s 1300mAh packs with minimal effect. There are also filters to help tone down some of the fumes/particulates. Even after the 15 batteries went up (in the mid ranged box, not even the large one) they still came up to the box and opened it a min or two later due to the dual walled container with insulation. Don't get me wrong, there is going to still be a lot of smoke, but almost remember that video linked above is a LOT of battery in those tests. That test is pretty skewed actually, when I ran the numbers I came up with this:
Test 1: 191.52W
Test 2: 403.2W
Test 3: 1394.24W (this was the massive fire)
Test 4: 151.2W shown plus "various packs"
Test 5: 201.6W shown plus "various packs"
Test 6: 215.04Wh shown plus "various packs"

The thing to remember is those were some very big packs. A lot of those are 4-6S lipo packs and double or quadruple the voltage we use in the tanks. A typical 6s NIMH pack is around 15-18ish on a stock 2000mAh pack. A 2s 5000mAh pack would only be 42Wh. That was a lot of battery they put in there and none of them were the same packs or amount of watt hours to make a good comparison. While I am a very big safety guy especially when it comes to lipo I do almost exclusively use them.

Here is the little mod my buddy made to his lipo box. It says on a welding tray in his AC'd garage. When he goes on trips he puts it in his smoker just in case it goes while he isn't there.

I like your friend's lined ammo box. That would contain most of the heat and fire. It would be nice to know what he lined it with.

5/8" sheet rock (drywall) is fire rated for a 1 hour flame exposure. That means it will take direct flame touching it for about an hour before it fails and breaches. The extra intense flames from a battery fire will shorten that time somewhat, but it will still take time to burn through. That's because the Gypsum in the sheetrock contains water molecules in the crystal structure that needs to be liberated (boiled off) by the heat, consuming much of the energy of combustion before it lets the flames escape. Two layers of 5/8" sheetrock lining an ammo can with a sheetrock lid that fits inside the closed ammo can would contain most of the energy of combustion. There would still be lots of very toxic smoke produced. That's my plan for a home made battery safe.

I've used Li-Fe batteries for several years with no issue. Lately, I've started using Li-Ions, and like them better due to the lower voltage of LiFe batteries vs. everything else.

FYI, I found a little 2s Li-Ion battery that is 3200mha, and very small (they'll fit in any HL tank). Price is $18(US). At that price, I can use 2 in a tank if need be, but I haven't really needed anything beyond one yet. If you are wondering....the catch with any Li-Ion is discharge rate. You can't pull more than 10 amps (which is why they aren't used for cars/trucks/airplanes or helicopters) from a Li-Ion. I checked my tanks with a power analyzer, and found they pull about 6 amps (max)...even for the 20 pound ones.

So, for me, they work. Cons are that they've got an odd connector on them, but one can buy adapters for a nominal fee.

I've been playing around with the 18650 battery packs also, but mainly to power my Open Panzer boards at ~12V. Thinking about buying some lower voltage packs for my other tanks with the 18650 cells.

He used hardie cement board and concrete patches for the corners.

After talking to him we are actually looking to test this. We are going to make a hardie board box and possibly test that ammo can as well I'll let you know the results if it is worth it or not...

I've read that some people use ceramic plant pots for storing and charging their batteries.

Yeah, I'm thinking of running some tests too. I have the advantage of being a firefighter, so I can have a crew and pumper stand by while I film it in regular light and with our thermal imaging camera. I could take temperature readings off the outside of the "box" and the smoke plume from a safe distance. I just need to find some Li-Po batteries to destroy...

If you and your friend want do some testing, I would suggest you contact your local fire dept. and explain what you want to do and ask for help. They can use it as a training / learning exercise and they will like that you are taking steps to minimize the danger they may have to face someday.

Let's coordinate when we do ok? I'm sure we can brainstorm up a few extra ideas....