I dont think you can use the words A frame and cheapest in the same sentence!!!
On my old humber 4.2m i used to have a pole lashed to the back of the seat with the nav lights and vhf antenna on - it was meant to be temporary while I got an a-frame made up at first but i got used to it and it became permanent so i saved the money i was going to spend on the a frame (£350 for a simple single hoop from humber) and used it for fuel.
This was the nav light unit i had on top of the ploe (but mine was black)