Eames 670 cushion repair ?  

Funny you should ask...
I just lightly restored a 70's 670(re-glued a shock mount,re-conditioned the leather with Pecard,replaced a leather button,polished the aluminum base,and steel wool-ed the Rosewood shells with Briwax)
I took the seat cushion to my friends shop,where he had a plastic welder.If you have the 70's version of the 670,it has the thicker polypropylene fiberboard backing to the leather.Anyhow,yes, I noticed many splits,especially around the front clips,as this is where the most stress occurs when you sit in the chair.The edges of the cushions curl upward when your weight is on the seat.Old vinyl and plastic stiffens with age, and instead of flexing,cracks,as is common in these seats.My friend carefully plastic welded these from the inside,so they would not show from the outside.This did involve unzipping the leather from the fiberboard.No small feat,as the zipper frequently jammed in the rounded corners.Best to have 2 people working together,one pushing the seams together,whilst the other zips or unzips.
Anyhow,after all this meticulous work,I gently placed the cushion back on the seat,where the curvature of the shell rolled the edges upward naturally as I slid it into the clips and,"craaack",the front plastic cracked again in the very same places!The down side is,when you sit,the cushion edges show a slight gap instead of "adhering" to the shell edges,as a healthy split free backing does.
The only other alternative is to obtain the backing fiberboard from Hume Modern or Graham Mancha,but the zipper would need to be stitched to your existing cushion,so I do not know what that involves.I guess if it's not too badly split,one can live with it,but it is annoying,considering the rest of the chair is near perfect!

fixing my plastic cushion base...
Thanks for your info. I tried to repair mine with a plastic glue used for styrene and added thin styrene pieces which were glued across the joints on the inside. I left it for a few days and when I flexed the plastic it cracked again. Do you know how much the new panels are? And are all the openings pre-punched?

Pre punched panels available from Mancha
But Zips need to be stitched on.I recall something like 35 pounds per seat cushion,but check his site to be sure.