You also might be able to use a portable grain auger to fill flat storage by making openings in the roof or by moving the auger around inside the building. A portable auger or a front-end loader can be used for unloading grain. Pneumatic grain conveyors also may be used for this. Because it's difficult to achieve uniform air movement in flat storage, it's difficult to dry the grain adequately. It's probably best to make sure grain is dry enough for storage before placing it in the building.
Perforated ducts placed on the floor can work well.
If the grain pile is fairly level, duct spacing should be about equal to the pile depth. If you have a long, triangular pile, you might get by with one duct, centered under the peak and running the length of the pile. Ordinary plastic drainage tile doesn't work very well because it doesn't have enough perforated area for good air movement. Try to keep duct lengths less than 100 feet to reduce problems with non-uniform air distribution. You can minimize condensation problems by providing plenty of air exhaust area and plenty of air movement over the pile while the fans are running, and by running fans often enough to minimize temperature difference between the grain and outdoor air.