I use clear acetate as the basis for making diffusion filters. It's tough but thin enough to be easily cut to shape and stuck to a wand for use like a dodging wand.
A quick-drying hairspray like Aqua-Net can be sprayed on to build up the desired amount of diffusion. It isn't messy like vaseline so you can set it down anywhere in your darkroom. If the diffusion is excessive you can quickly rinse off the hairspray in water.