I believe you can make the reservoir work, but you are paying for features with the XT that will not function in the horizontal mode.

1. the "fountain effect" is gravity driven and will just flow down the top with no visual effect.
2. the "fill level sensor" will not function properly because the water column measured, if any, will not reflect the water volume.

If you use a reservoir in the horizontal mode you should be able to seal it so it will not leak, but you should have the suction port rotated to the lowest possible position, and run it as full as possible.

I believe the deflector plate is for the case of not using a bottom port (not fountain effect or a top port) for a return, which reduces vortexes & air suction into the supply port.