In truth, your best bet without adding another fan would be to move that top fan to the top front location as a second intake, then, technically, you would have the fans in the correct places and technically the correct orientation for positive pressure. You won't actually HAVE positive pressure because with no fans in the top and an open grill all your intake pressure will bleed off through the holes and be vented by the rear exhaust fan, but you will at least have better air movement directly to the CPU cooler and proper orientation.
Later it would be wise to probably add a top-rear fan as a second exhaust, giving you "technically" neutral pressure, which is what you really kind of want anyhow. When cooling performance is not the biggest concern because you have more than adequate cooling, then you can worry about positive pressure/dust suppression. Otherwise, negative pressure is technically the configuration that gives you the most cooling performance. In ANY configuration however, direct ambient air which is cooler, being supplied in a straight line directly to the CPU cooler and over the top of the memory and VRM components on the motherboard, is the MOST important consideration. Putting that fan in the top front would give you that.