I used to use rubberized cloth. It's cheap. It keeps the guitar from sliding around. It doesn't fix the odd sitting position. It left residue on my Martin guitar's finish.
Then I tried this for a short while: viewtopic.php?f=129&t=107244
It was good but not great. I can sit in any chair as long as my thighs are perpendicular to the ground. My back is straight. My knees are even. I have far less fatigue. No more back problems from sitting in an odd positions for too long. The support would collapse after some playtime, probably due to vibration. It never really felt right. It is not as stable as I would have liked. I was afraid of damaging my guitar by over tightening. You can use it for multiple guitars, but it has to be readjusted each time. No more foot stool! I have a friend who teaches guitar at Indiana University who swears by his Murata support.
I now use a Barnett support: Mod note: Commercial link removed.
I can sit in any chair as long as my thighs are perpendicular to the ground. My back is straight. My knees are even. I have far less fatigue. No more back problems from sitting in an odd positions for too long. The guitar doesn't slide around. Extra sets of magnets allows the use of the support on several guitars. No more foot stool! I did have a problem with the support losing its position after an hour or more of playing probably due from vibration. I fixed that by fitting the above mentioned rubberized cloth between the two long adjustable supports and adding a lock washer to the tightening nut. I have been using it for months now. I'm very happy with it. If you're not handy have someone who is install the interior magnets for you. The directions are clear. Everything is reversible.
If money is an issue go with number 1. If you're afraid of or don't want to bother sticking things to the inside of your guitar go with the second choice.
I know there are other choice that either use suction cups(no way!!!) to attach the support or fits on the end pin of a guitar. Classical guitars normally don't have end pins. So if you go with the latter choice you'll have to involve a luthier.