Smith and Wesson
what are the inside mesurements
I need the box to be 20 long x12 high x10 wide
Drill holes with a dremel. Run rust-proof wire or other types of anchor through these holes. I plan to bury mine a bit as well so it's not just sitting on top of the dirt. You could also on the inside attach something heavy like rocks in a plastic bag so the rock weighs more. Maybe with duct tape
Once it were staked down children could sit on it. It is not really designed for that purpose.
Yes, Home Depot has four different rocks, this one included, at different prices and sizes. I bought all four and am going to make a rock garden out of them. I used them one rock to cover some recently installed clean-outs.
The manufacturer will recommend a particular "rock" if you tell them the size you want to cover. This one would have covered that size but a smaller one would have also.
Yes metal rods. comes with 4
Yes, it is quite easy to cut. We used a jigsaw to take an arc of the back off for the purpose you mentioned. .
Very little in itself but you could insulate around the valves and cover the whole thing with the rock.