How to Make a Concrete Box

Cut a plank of wood to the same length as one of the sides of the concrete box. Repeat this three times to create four planks. Nail the planks together in a square frame; this frame should be the same size as one of the sides of the concrete box.

Seal the corners of the frame with silicone sealant. Place the frame on top of the sheet of plyboard. Seal gap between the bottom of the frame and plyboard with sealant. Put on your dust mask and goggles. Open the bag of cement and read the instructions on the side. Add the required amount of cement dust to the required amount of water in a bucket. This may vary between types of cement so read instructions carefully.

Add the rough gravel to the mixture to thicken it and strengthen the resulting concrete. Mix the mixture evenly with a stick and pour into the frame. Use the trowel to smooth the mixture in the frame and then leave to set.

Test the concrete by touching it with a finger. If it is set completely solid then it is ready to remove from the frame. Use a chisel to chip away the sealant and excess concrete and remove the frame. Move the concrete slab to one side and repeat to make five slabs.

Press one slab up against the base of another to make an L shape. Pre-drill a hole through the base of one of the slabs into the end of the other slab with a masonry bit. Then use an electric screwdriver to screw a concrete frame screw into the hole, securing the joint. Repeat this with until four screws are screwed along the base of one slab into the end of the other slab.

Repeat this with two more slabs to create two L shapes. Put these shapes together and affix with a masonry drill and screws to make a square concrete frame. Lay the remaining slab on top of the frame to close the base of the box. Affix this with four lots screws at each edge of the slab, making 16 screws in this slab alone. The open-topped box is now complete.