When I wanted to move my solar panel from the deck to the stern railing I looked for some time around to source the most economical rail mounting brackets. As usual, if marketed for boats they come at a premium. The cheapest I could find was £4 each if made in plastic and double that if made in metal.
I finally came across "spacer bar saddles". They are used to fix tubular cable trunking onto walls. They come in 20 and 25 mm diameter, in plastic (white or black) and in metal (zinc plated). And they are really well priced, usually from £0.20 each in plastic and £0.30 in metal.
Here is a close up of one:
I've covered the screw that holds it on the frame with insulation tape and will also put some on the rail as not to scratch it.
Here they are on the back of my panel :