Concrete and PVC
First couple of years I used removable medium but never needed to adjust the weight, only needed more, no regrets with the concrete.
4 pcs of 1.5 inch, 1 pc of 3 inch and 1 pc of 4 inch schd 40 pvc.
Only regret was using plastic pipe, even as the crete was curing I was thinking if one of the plastic pipes breaks, I'd be stuck with a much shorter hole, I would use HD steel next time.
I did not add a receiver to the box at the time, after giving it some thought I figured if I wanted to tow a trailer I wouldn't want all that extra weight. After giving it more thought I still don't think I would do much trailering with all that weight but just for other misc insert use it would be handy to have one.
IMO the bottom of the box is to low for the receiver, I'm thinking now of the top, which is probably a bit high for trailer hitch, without a drop, but the right height for other uses.
JB.