I always used to carry a chunk of 5/8" dowel. You can screw a mic clip onto it for plant mics. Paint it if necessary, fits in a C-stand knuckle, nail it up if you want. Cheap, light, and disposable.
I also carried a putty knife mounted baby spud. Jam it in a crack in the set to mount a plant mic. Helicopter clip baby spud for dropped ceiling plants. And, of course, a knuckle to put on the baby spuds to hold the 5/8 dowel.