The StreetDeck trial not only lets you try out StreetDeck, it also includes the developer tools which will not timeout, for free.
The developer tools are all contained in the development enviornment which is used to create both new skins and new DigitalMods (Script Projects) for StreetDeck.
To help you get started making your first StreetDeck skin. Check out this
tutorial video (75 mb, 18 min) which walks you through the basic process of creating a new skin. Note, this video is a bit out of date and doesn't include some more advanced skin editing topics like templates. Some of the menu items in the development enviornment have changed, but the basic concepts are the same.
Also be sure and check out the StreetDeck developer sub subforum for some more documentation on developing for StreetDeck.
The skin shown in the tutorial video will be released later this week.