Profile Design, make a double bottle cage called the Aqua-rack that mounts to the seat post. I have this on my Tri bike. The fit is pretty snugg, so I recon they should hold while MTBing, provided you are not doing any major drop-offs or jumps. http://www.profile-design.com/nice_assets/0000/0388/aquarack_pu_389.jpgEnlarge image < ="application/x-shockwave-flash" ="http://www.pro-design.com/s/layout/supertext-02-bold.swf" quality="best" flashvars="=Features&width=387&height=13&fitexactly=&tunewidth=&tuneheight=0&offsetleft=&offsettop=&thickness=&sharpness=&kerning=&gridfit=pixel&zoomsup=false&flashs=&opacity=100&blendmode=&size=13&zoom=100&=.sIFR-root%257Bfont-weight%253Abold%253Bcolor%253A%2523FFFFFF%253B%257D&able=true&lines=1" wmode="" ="#000000" name="sIFR_callback_1" id="sIFR_callback_1" allowaccess="always" sifr="true" height="25" width="100%">Features 6061-T6 aluminum clampand extension Includes 2 lightweight Profile Design� KagesT Fits 26.8mm to 27.2mm seatposts Matte black