The CO2 setup will just fit under the right edge of the HOB filter if you place it all the way against the right wall in the back housing of the Edge and place the HOB filter as far to the left as it will slide. However, you have to cut a notch in the cylinder stand to get the setup to fully snug up against the side wall because of a stiffening rib in the base. This probably sounds pretty confusing but hopefully the pics below make it clearer.
This picture shows where in the base of the CO2 cylinder should sit (what you're looking at is the rear section of the plinth that the tank sits on from above the tank). Note there is a notch you need to cut in the plastic cylinder base, highlighted by the arrow. Also note that the cylinder base has 2 flat sides and you want one of those sides to go against the wall.
Detail pic of the notch. The plastic is really soft and cuts easily with an Exacto type knife.
This is what the final positioning looks from the top.