Easiest way to remove trawlers from an area is to remove the area they fish......artificial reefs are not trawler friendly.
Start by using old concrete pipes, bridge beams, pilons, block work in large chunks and scatter them randomly over the area you want to protect from nets. Over a decade you could have a massive area that is trawler free and a growing ecology.
How many millions of tonnes of concrete and bricks are used in landfill each year??
I just sent to scrap recycling about 6 tonnes of large steel pipes that would have been perfect for an artificial addition.