I have a trunk mount tank and even that cooks off some of the alcohol due to high temps in the summer. (you can smell the alcohol smell when you get in the car after its been parked in the sun).
Temperatures near the fire wall after the car is parked can get very high up about 200 degrees on the turbo side about 30 minutes after you park ( I melted a themometer lense that was rated to over 200 degrees)
On the drivers side (opposite turbo) temps are much lower.
I have also run a reservior in front of the battery at the very front of the engine compartment where it stays relatively cool due to air coming in as your driving. That seemed to work well too.
One key is to keep the resevoir small enough that you are not leaving the mix in there for a long time before it gets used up. The biggest tank I have run only holds a liter and that is good for 2 tanks of gas if I'm just driving around town, or a day at the drag strip if I'm flogging it hard.
The 1 liter tank cost about $5.00 at NAPA, the front of the engine compartment tank was an old empty coke bottle (ie free) -- pay your money and take your choice.