You want the stopper on the tank to face FORWARD.

The tank mounts in front of the engine in the fuselage... usually straddling the C.G. by some amount.

You can either stuff foam into the fuse and slide the tank in, then put additional foam to pad it into place... which some people do...

Though I don't recommend ...

Or you can make a retainer from small light ply pieces of wood.

I usually do the latter.

I glue foam between the ply and the tank ( on the wood side ) to prevent transmitted vibration, and to absorb shock.



BTW: Many ARF's leave this up to you. My H9 Funtana S90 came similiarly set up... no retention for the tank.