Thank you for your inquiry regarding this item from Prime-Line Products. No, you would not be able to substitute a round for a flat spline. There are a few flat splines to choose from. You may wish to contact Prime-Line Products directly by calling their toll free number at (877) 649-9740, or by contacting them by email at email@example.com where a Product Specialist there can better assist you with your inquiry. You may also send photos in for us to visualize precisely what you are inquiring about. For faster service, be sure to take advantage of the CHAT option located in the lower right-hand corner of your screen, where you can quickly connect to a representative for assistance.