these are shaped the way they are for a center overflow, but just one threaded connection per overflow is how i make them for square tanks. Notice the threaded connection for the majority of the length of it, allows you to easily change the angle if you want.
How big are the pieces of pvc you re using to lift your rock??
acrylic rods are a much better way to lift your rock than pvc if you truly want free flow underneath, as well as are more attractive. Just take 1" plugs 1/2" deep and drill a 3/4" + a touch hole in them and glue them to the bottom of your tank. Then cut your rods however long you want your lift to be and slide them in. You can have different length rods and interchange them for specific rocks as they just sit in place. The amount of rods required depends on how large your rocks are. I just space them evenly 6" apart for the first row, then the next row 4" behind that staggered etc etc. and take a grinder to the tops to make it grip.