Tell you what I used to do:
Arm yourself with a bucket, pop into B&Q and get some of those water absorbing crystals. They come in a container that you place inside the bucket. Leave the whole asembly in the a/c, not forgetting to empty and leave behind when aviating.
I was amazed at how the condensation disapeared. The instruments and avionics where condensation free.
PS
And, I should also say that I have nothing to do with....Oh ****** it, you know what I mean.