I found these plans online and was just waiting for the kids' grandpa to come out and build it. Could be built in one afternoon (depending on how many trips to your local Home Depot, and availability of the correct tools)
Modifications from original plan:
6x6 instead of 4x4
Brackets to hold patio umbrella for shade
Added seat gussets for additional support
Added "wooden flange" around bottom of box to run lawnmower on
One additional center back support on each bench
One centered handle (instead of a handle on each end)
Stapled landscape weed barrier fabric onto the bottom
Sanded all exposed edges
Materials:
4@2x10x8'
12@1x6x6'
2@2x6x12'
8 hinges
2 handles
3 1/2" decking screws (for attaching 2"x boards)
1 5/8" decking screws (for attaching 1"x boards)
2+ 1 1/4" 2-hole straps for metal conduit (for attaching patio umbrella*)
*Our patio umbrella pole was 1 1/2" diameter, but 1 1/2" straps had too much play, so we used 1 1/4" straps and misc flat washers to adjust the snugness to the pole
Cut lengths:
2@2x10 cut to 72" (long sides of the box)
2@2x10 cut to 69" (short sides of the box)
Cut one each of the following out of a 12 footer:
2@2x6 cut to 75" (long side "flange")
2@2x6 cut to 64" (short side "flange")
No cuts were made to the 12 1x6's
Used one leftover 2' from the 2x8x10's ripped in half cut to 11.5" for arm rests
Used two leftover 2' sections from the 2x8x10's ripped in half cut to 21" for back supports
Used the last 2' cutoff to make two seat gussets (cut about 9 1/2" off and then ripped it at 45 degrees)
Used the remaining piece (ripped in half length-ways) to attach to the center of the backrest for additional support and as the attachment point for our handles
We decided to do one handle in the center of the backrest so one reasonably strong adult could open the lid
The bottom "flange" was assembled with the box upside down. Basically a 2x6 screwed to the bottom edge of the box allowing 1 1/2" overhang all the way around
After making cuts, but before assembly, we recommend using a quarter-sheet pad sander (or similar) with 60 grit paper to break all of the edges and round over any ends or corners that will be in contact with sandbox users
Followed other general assembly instructions
