There are lots of how-tos like this but they all seem to involve padded seats. This is a bare bones file crate seat that you can drag out when roasting hot dogs around a fire. The seats are cheap because you can use garage sale crates and recycled wood.
Plastics file (milk) crate
about 6' of recycled wood (mine came from an old pallet)
Tape measure & pencil
Electric drill and bit
Drill and countersink bit (and a screw driver if you don't have a Phillips bit)
1-1/4" Phillips head wood screws
Router & bit (or a rasp)
Sander (or just sandpaper)
Varnish or paint & brushes
Step 1: Measure, Cut, Assemble and Finish
There are two layers to the seat. The top one is what you sit on. The lower one , perpendicular to the top layer, supports the upper one. They are held together with wood screws.
Measure the inside dimension (ID) for the lower supports over the "ledge" inside the top of the crate. The dimension in the photo is 13". Cut your two support boards to 12-3/4". Measure the crate ID in the other direction to find that dimension and, again, deduct 1/4" for the outside edge dimension of the supports.
Measure the outside dimension (OD) of the top of the create. Add 1" to each dimension for the total length and width of the seat. If the seat width is to be 15" and you have boards 5-1/2" wide you will need to rip one to 4" or less. If you rip the board to 3" you can leave gaps and route the upper edges between the boards as well as the seat ends and sides. Making sure the bottom supports will fit inside the create and the upper seat will extend over the perimeter of the top of the crate, you can cut the boards.
Lay out the seat boards, bottom side up, on a workbench. Using the inside dimensions, mark where the support pieces should be located. Verify all dimensions because the next step is assembly. Using the correct countersink drill bit for the screws you are using, drill two holes through each support board for each seat board. For 1-1/4" screws drill the holes about 1" deep. Assemble using the screws.
Secure the assembled seat to the bench and route the upper edges or use a rasp to round off the edges. Sand and finish. Be sure to sign and date your project on the bottom.
If you go camping or on a picnic you can fill the crate with supplies and sit on the seat when you get there.