About to get an AEM methanol kit, and I fully realize there is 0 extra room on my 03 R53. I really don't want to get a mini battery and mount the 1 gallon tank in the back of the car, so let's pretend that isn't an option.

Is there any other sneaky places I could mount a 1 gallon methanol tank? How much volume does the head light washer tank hold?

 

Keep a full size battery and mount it in the trunk....for a full gallon...not many choices...
You will be carrying spare bottles of meth anyway in the trunk.....
For smaller tanks...
You can mount them in the trunk sidewall (behind plastic side panel)...but limited space, hard to get too, messy to fill....
Other option is to put a tank up front....passenger side....by the abs/DSC pump by the firewall....
Other option is the washer tank...pretty small too...
How about making the tank look like a sub for a radio? Carpet it.....having it IN THE CAR on a hot day...is less than ideal...actually a fire hazard....and may vent on a hot day....but you mentioned the tunk (battery).

 

Hmm. I see, I may need to get creative and report back.