I use PVC pipe fittings. I take a PVC T and cut off the back half of the
through line. Actually, by cutting a little less than half, the T will snap
over the leg of Rohn 25 with the 90 degree line pointing away from the tower.
Then add another T to a 6 inch piece of PVC pipe and insert into the 90
degree line of the piece on the tower. The outer T should be vertical. I then
run the ladder line through the outer T and tie it with nylon cord. I also
add a stainless pipe clamp to the PVC where it snaps onto the tower to hold
it in place. You will need several of these standoffs depending on how high
you are going.
Once you get the design you like, you can make them up and glue all the
joints while on the ground.