hi,
i made mine from scratch. as u said, thinnershelves tend to vibrate and reduce sound quality. therfore if u use a sturdy shelf,the bass is more fuller and better quality, as the shelf is not absorbing all the bass vibrations. If ur gettin a sub and are only using the 6x9's for volume,it might be ok to mount on original shelf as u prob wont notice the difference. if ur only putting in 6x9 for the minute,is def best to build a shelf from scratch with thick mdf wood.
i used 18 mm mdf,which waas sturdy,but heavy. the bass from the speakers sounds clear and deep and not flimsy. im sure that 15mm shelf would be alright,but would not recommend anything thinner than this,as becomes very flimsey(u can tell by holding the wood by the sides and pushing it in the middle).
Im not to sure about re-enforcing your original shelf,as if you wre to go into all the trouble to do this,might aswell make a new one. all u need is mdf wood, a jigsaw, a pencil, spray glue(to stick the carpet to the shelf), and some carpet that matches your shelf. the glue u can get from maplins or alternative, the carpet from your local ice shop(they might be able to give u half a roll,like mine did, as thats all u need to cover the c's shelf),otherwise halfords do it.
hope this helps.
oh you need quite a bit of time and patience too.