I built a pool side box of Cyprus for daughter #2
It is currently about 800 miles from me at the moment so the following is my recollection. 39"w x 35"d x 36" h.
3/4" slat sides that are mortised into the four legs. 3/4 " thick slats x 6" tall. Full thickness mortise x 1/2 to 1" deep.

legs are approx 2" square.
top & bottom rails are pocket screwed into legs. Has 5 hinges from Rockler  that are "anti-slam " type
http://www.rockler.com/lid-stay-torsion-hinge-lid-support-rustic-bronze
50# per hinge b/c the leverage of the top is so great & the focal point is so far back the top becomes front heavy

The PROBLEM:
when you raise/lower the lid the top back rail twists. Torques.

Possible solutions:
(a) mortise the back upper rail into the legs
(b) somehow connive/obtain a different setup that  will slowly allow the top to close.

Questions:
Can I mortise the upper rail into the rear legs w/o breaking the cheeks of the mortise. How thick should the tenon be & how deep into the 2" square leg can I make it? to avoid twisting.
Must I make the legs 3x3, 4x4

Any & all recommendations appreciated

thks 

smitty

are the slat mortises free, glued or mechanically fastened??

WTB your pocket screwed joints have loosened...

the hinges are not in sync for operating load/tension...

the lid was closed by pushing down on a corner and the the lid acted as a torquing lever against it's self...