Please post a pic of the exit. What needs to happen is the air should be drawn through the cowl and out. If you don't have enough exit area there will not be a partal vacuum created at the exit and no flow past the engine. As previously stated, 2 to 1 exit to intake is needed. The size of your inlets should be fine as long as you get airflow. I once made some baffles for an SBACH by carving some ovals out of foam and then covering with packing tape. I waxed the packing tape and laid some glass cloth and epoxy resin over that. Once cured the part can be split and peeled off the foam. I even tinted the resin red to match. In the end it only dropped the temp 10 degrees. This was on a twin BTW.