I'll probably have a look at it sometime (as soon as I build up some confidence! Actually I thought it would have been a simple job of removing the air box and clamping on the cone filter, but I guess not). Also where is the air intake temperature sensor located?
One thing I noticed is that the cone filter (that was bought for me) is about 3" in diameter, whereas the intake hose (between the air box and the engine) definitely looks to be less than that (maybe 2.5" in diameter, guessing here, didn't measure it), so that'll be another thing to overcome.
Maybe the adapter for the Sentra is shaped differently. Actually have a look at
this (just pulling random images off the net here). It seems that sensor would be bolted onto the adapter and then connected to the air intake to the engine. I suppose the Echo's sensor is made up differently (haven't had a proper look at it as yet).