The one and only true MJ storing device is the kinesis pipe.
This is a natural result from the way, MJ works. The machine requests a package of energy from its adjacent pipe. If the pipe has enough energy, it gives it to the machine and requests this amount from the next segment until there is a wooden pipe sucking this energy from its engines. Its much like a snail "walks".
To achieve this, each section of pipe has an internal buffer storing as much MJ as its potential troughput (e.g. cobble stores 4MJ, quarz 64) (CJ states this in his video for the iron kinesis pipe).
So since pipes no longer explode on overload, you can just form a loop of diamond kinesis pipes and there you have your storage (1024 MJ per segment).
Its a very impressive look having the whole loop full of the glowing blue kinesis energy turning red now and then