This is 2nd hand info, but I group I know showed up to an enduro w/ home made ones once. Said they worked well. The took a pair of 100% cotton t-shirts & put them one inside the other, then looped tygon tubing around, held it in place by running seems of stitching around (careful not to pin hole the tube). They used a 6-pack sized cooler & something like a windsheild washer pump to circulate it.

They said it worked well and was pennies on the dollar compared to buying it, however I suspect it was an experiment for a particular 12 hour race, don't know how many times it got used.

We use bicycle water bottles in enduros & find that even water warmed up by the transmission tunnel helps noticably. Yes, as Jake says, pay attention when taking a toke on it. I almost went in the turn 1 gravel once due to coughing so hard my eyes were watering after pulling really hard on it & getting more than I bargined for.

Matt