on OE fan you can just mount it upright along side of the fan securing the two hangers into the side flange of the fan housing , I had it this way before going dual fans
with dual fans I mounted it laying flat with the hanger screws underneath and the hangers pulled to the sides secured into the fan housing just above the passengerside fan , only problem i had wasa little coolant blow out throught the vent holes in the bottle neck , i plugged those and run it with minimal fluid.
edit ,
just realized you were installing on a TTA which explains why the tank wouldnt fit there on my GN when i was told told to put it there , in your case ignore the above, but for regals it works