Joan, the A590IS is good value and will cetainly do the job, be patient with the poor lcd screen. AS for a screen cover all you need is a bit of stiff card, cut to shape, of the right thickness so when it is bent ti fit over the screen it will slide into the gap between the camera body and the reversed lcd screen. The benefits are that it costs nothing and will do the job, no need to spend money on some thing like Rick's contraption. As for an an adapter, you have to decide do you want to look through the scope as well as take photos, in which event you will need a swing away adapter on which the camera can be fixed or one that can be put in place and removed quickly. It will depend on what scope you have, I do not recall this being mentioned. The SRB mk11 swing away is excellent but may not neccessarily be the thing for you, however have a look at the opitcron adapter that can be fitted to the eyepiece and removed with little effort but for that you will need the appropriate Canon LA... adapter for the 590.
Robert