The most useful accessory need not be the most expensive - dust caps!!
Last week I was out with the Celestron RASA 8" (under very poor skies unfortunately) and whilst setting up the whole telescope and camera system, I misplaced the camera nosepiece and its dust cap. At the time, to stop dust getting onto the sensor optical window (causing nasty black spots or dust doughnuts) I simply placed the camera face down on the table.
However, I realised that in my "box of bits" I had some Baader Planetarium male T-2 dust caps (part of their T-2 system).
(Unused) Baader dust cap in its bag
These caps, which are made of high quality black plastic, weigh about 7g, and are ~50mm in diameter. The outer edge is knurled for easy gripping when attaching or removing the cover (see below).
Baader dust cap showing its size
The caps simply screw into the "front" body of the camera where the nosepiece usually "resides" and which, like many cameras, have female T-threads.
Camera sensor optical window exposed to the elements
Once screwed in to the body of the camera your optical window is covered, protecting it from dust (below ; you can of course screw it into a 2" T-2 female threaded nosepiece too of course).
Ah - now the cap is on covering the optical window
Such a simple, and inexpensive accessory, can be really very useful and can save the camera sensor optical window from becoming dirty/dusty. These dust caps are available from your local/favourite astronomy retailer - (see >>here<<) - and ask for Baader product code #2904000
Oh...if you are wondering if I ever found the nosepiece and dust cover....I did a few hours later....it was in my pocket all the time.