A soft box safe for jumping
Plyo boxes are usually either made of metal or wood, which means when your jump goes wrong, you could get injured from contact with the hard surface or sharp edge of the box.
The A-Unit Plyo Box removes this possibility as it is made of foam that's soft enough for a bad landing and yet firm enough to provide stable landings.
It allows you to jump all out without fear of not making it. You can also use it as a bench. The boxes come in sizes of 6", 12", 18" and 24", which can be used separately or joined together by Velcro.
The A-Unit Plyo Box is no longer available for sale but there are a lot of alternatives out there like the CFF High Density Foam Plyo Box from Amazon. Other options are pretty good too.
Hard wooden or metal boxes aren't safe. Most gyms have stopped using them and opted for soft plyo boxes instead. Jumping helps improves your explosiveness and strength.