I've had good results from the Acerbis 3l and 5l tanks that attach to the forks. You can also get a siphon kit for them that attaches to to the main tank's breather. I don't bother with the siphon kit but keep a short length of hosepipe in my backpack and use that to siphon from the header tank to the main tank.
I contacted Greta at Acerbis in Italy to find the local importer (greta.m@acerbis.it). The price is about 60euro.
The photo shows the 3l tank.
For me, trials bikes are the answer in the mountains, I highly recommend one.