yep what the others said .
but if you want cushion , go get a piece of horse hair saddle blanket or some felt carpet pad . You want something at least ½ thick .
Slip your finished Moc’s on and stand on the pad . Trace your foot outline and then cut to the inside of what you trace ..
Slip the cut out into your Moc and you now have at least some padding .
Frankly that’s what I do in all my Moc’s anymore .
Now the problem
Depending on how you actually set your foot down when you walk , you probably going to find that your center seams will roll a little after they break in .
Case in point . The seam on my right foot stays mostly center . But my left foot rolls alittle when I walk and thus the seem curves to the out side .
This roll causes the insert to sometimes drift with that roll . So I have to stop , take the Moc off , slide the insert back down and then go on about my day ..
the same thing happens with a thicke piece of leather if put on the inside . for me it still rolls and i end up walking off the side of it .
Speaking from experience , you also don’t want to glue it in.
It will just come lose and ones the insert wears , you then feel the glue , even if you use rubber cement