I use a 20m delta loop in my small urban back yard.  It uses DX wire (a black PTFE insulated wire) to be discreet, and works okay.

That said, I originally just put larkshead knots (aka “cow hitch”) in the corners, and tied those down directly to spread it out.  A few months after install, I noticed that the pressure from the line in the corners had caused the PTFE to creep such that the wire was exposed.

This model was made to provide a nice, pleasant path for the wire to take, and to keep the tiedown outside of the loop.  This way, if the wire pops out of the holder for any reason, it will fall down, and not incur more damage.  A small length of string is used to lash the wire down into the holder (I was worried the PTFE would just creep its way out of the trench).

This was up for a couple of years with no real issues, though I had to take it down for house painting a few months ago, and haven't gotten the antenna back up yet.