I posted a few pics in shiv's 91 octane dyno thread but didnt want to put any more info in there.
I am ordering an Aquamist HFS6 meth kit for my M3 and have been researching tank options.
so far here is the best option that I have found that really helps tidy up the install process, wiring, tank, pump, etc.....its also not cheap
They have several different options depending on your space constraints and capacity requirements.
take a look: http://www.howertonengineering.com/tanks.htm
Their biggest 2 gallon twin tank design costs $319...it goes down from there.
example of interals
I know most of you are not to this stage yet w/ Meth injection as you are waiting for a tuning option, but you can use this for future reference.