An ideal place would be where the structure gives good change of depth and acts as a funnel or highway from the spawning/feeding area to deeper water in water about 6 ft deep (2 ft under and over the structure). This all assumes you will not be using pelagic fish for forage (shad/herring) in which case other options may be better.

The ditches and humps you have are structures. You don't want to fill in the ditch. You could put them on the top ditch edge to act as a further funnel/irregular feature. Sunil's suggestion of putting them in an area with no features (flat bottom) in groups is good. Just think about water depth and the possible thermocline. For the part of the year when the thermocline exists structure below it will not be productive.