[quigly]

I wasn't able to find the 12 inch version, but here is the 9.8in version.

http://www.redwolfairsoft.com/redwolf/airsoft/Accessories_Conversion_Kits_AEG_Madbull_JP_Rifles_Mid_Size_9_8inch_Handguard_w_1_Barrel_Length_for_Marui_M4_series_TN.htm

 

[quigly]

Any of these kind of lipos should work for you. http://shop.ehobbyasia.com/bb-battery/batteries/lithium-batteries/kong-power-1300mah-20c-continuous-11-1v-lithium-battery-mini-tamiya.html
NOTE:some buffer tubes are to short, causing full collapse inoperable. And stick type(such as for AKs) will not work due to the long size.
Also, I don't believe you will be able to fit a plug and play mosfet into the buffer tube as well. You can always wire one into the GB its self, which may be a better idea all together as to minimize any shock it will take from running with the rifle.