Had the same problem some years ago, till I got some "brush" seals on the doors of the obsy. (Cut the wasps back, but not totally) Ordered a couple of these re-usable shower caps to try. They're made of a "silk" type rayon, come in different colours, and are double layered. Quite durable. A wide, adjustable elastic with buckle. Problem solved, and at $2.65 each delivered, ex PRC. I ordered another 10 for my other scopes - even if I have dust caps on, these go on top.
Just covers a 12" newt, but also use them on focusers on my RC's and refractors.
