I think I completed all the icons for the current built in applications of nemo. Here they are.
There are three types of icons in this iconset. The first, and preferred one, if you can manage to do it, is to find an object representative of your app that is circular, like the dialer, or the combination locker. It doesn’t have to be absolutely circular, some parts can stick out of the circle and actually this practice is recommended.
The second type of icons are objects that are not circular but can be easily recognized even if modified to a circular shape, like the sticky notes, or the packages. This can be done by either cropping or reshaping the object.
The third solution, least recommended but always easy to do, is to just paste any icon over a colored circle. This is actually recommended for logos of established brands/apps who do not allow their logos to be modified. The circle has a radial gradient from the center to the rim, with the lighter part at the center and maximum difference in lightness between rim and center at about 50.
Here is the calendar icon in svg to base your own icons on it. You will need the open sans font