We've all been there. Running on muffs & waking up everyone within a 10 mile radius of the engine or p!ssing off the people enjoying a nice quiet pint in the pub on the front. I have solved the problem!
Take 1 bit of plywood slightly bigger than your prop hub diameter, 1 bit of bungee and an old cloth / rag.
Dril la couple of holes i nthe ply and use the bungee to hold the ply against the exhaust hole (hook it round the font of the leg above the gearbox- it wonlt fall off), bunch the cloth up and put it in between them. This essentially simulates the prop being underwater, and so the exhaust is forced out the idle ports via the silencer the way it was designed to.
The cloth allows the exhaust cooling water out & acts as a sound muffler.
Don't just use ply - it will rattle against the prop hub with the exhaust pressure trying to push it out thre way and double the noise levels!