By the way, since I mentioned there's a better and easier way to draw the legs and the lid, here's a tutorial of sorts.
Start with the front left leg. Draw a slightly over sized blank. Draw the curve for the leg on the right side of it. Select the top front and left edges, shown in blue, below.
Get the Follow Me tool and flick on the waste side of the curve. Then triple click on the leg and make it a component.
With the leg selected, get the Move tool and hit Ctrl on the PC or Option on Mac to invoke Copy. Move a copy of the leg down the red axis to make the front right leg. I used the Tape Measure tool to set out guidelines showing where the inside faces of the legs need to be. Right click on the copy of the leg and choose Flip Along>Component's Red to make a mirrored copy of the first leg. I put some marks on the top of the leg so you can see that the leg isn't merely rotated. (In this project rotating the leg 90° would work but often rotating creates issues so you might as well learn to do this the way I show.)
After the right front leg is flipped and in position, select both front legs and use the same process to make the rear copies. This time right click with both the rear legs selected and choose Flip Along>Green Direction.
Use the Rectangle tool to draw the cross section dimensions of the box side. I drew it using the top inside corner of the front left leg as the starting point.
Use Push/Pull to extrude the rectangle over to the right front leg. Triple click with Select on the long box side to select it all and make it a component. Then use the Move tool to move it forward the desired distance. (3/8 in. in this model) Copy this box side to between the rear legs with Move/Copy and flip it along its green axis. It wouldn't look any different now but if you do something to modify the box side later, both sides will get the same treatment in the right location. Repeat the process for the short box sides only flip the right side compoennt along its red direction.
Now, for the lid. Since the lid is rectangular and the curves meet in the middle, they curves have to be different on the long and short sides. The easiest way to handle this is to start with a square. delete its face, and draw a profile or cross section of half the lid. Then select the edges of the square.
Get the Follow Me tool and click on the profile.
Triple click to select all of the lid geometry, get the Scale tool and grab the center handle on the end.
Move the cursor to the outside edge at the top of the right leg to make the box lid the right length.
While the lid geometry is still selected, hit G to create a component.
It still needs a bottom. And if you are going to use mortise and tenon joinery and want to show it, you could edit the components to add those details.
One key thing to notice here is that I drew everything in place. This eliminates the need to know all of the dimensions when you draw the model. If you place the legs correctly in the beginning, then the box sides must be the right length since they were drawn to fit between the legs. And the lid must be the right size for the same reason. Less room for errors and you can let SketchUp tell you what all of the dimensions are.