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Sometimes the obvious eludes us. For years I covered the diagonals in my scopes with plastic baggies. They'd blow away, get lost, rip, you name it. Finally, one day it occurred to me while holding an empty tin can that such a can of just the right size could be used to slip over a round diagonal holder and keep the dust and dirt off the mirror. Unlike a bag it wouldn't blow away or rip and would be substantial enough to remain around for a while. I rooted around and found an old straight sided plastic bottle just a bit larger than the diagonal holder for my 2.18" secondary mirror. Thirty seconds with the band saw and the top was cut off making a plastic version of the tin can. I got out some 1/16" black felt and found that by lining the interior of the bottle with two layers (held on with double sided tape) it would just slide over the holder and stay put. The bottle was spray painted green (I had some green spray paint hanging around) and voila! - a super little diagonal cover. I have another one made out of a 2" diameter fruit juice can. Cost for this project, $0.00. A final wrinkle might be placing some desiccant in a bag at the bottom end of the can so after closing up for the night any moisture on the diagonal will be absorbed and the surface kept dry.