None. I've never felt the need to with a crew cab or extended cab truck, especially with new tires. On my '13 crew cab 6.6 bed when I first bought it, I went a whole winter with bald 60k tires and never had any problems getting stuck with 4x4, not to say it was safe though.

New winter tires for my stock wheels and I don't think adding any weight is necessary. Whatever piles up in the bed of my truck is good enough for me

 

That’s a nice box

 

About 300lbs, I ski so I want the snow, have driven 2+ foot deep over back mountain roads.

4x6x3/4" horse stall mat stays in year round, helps deaden bounce. That's about 100lbs
Similar rubber 3x3 mat's probably another 100
and then 2 tube sands 60lbs each