O2PE includes an unwrap function (Surfaces → Unwrap Structure …) which should work pretty good an simple structures.

I use it this way:

• Select the structure you want to unwrap and change to top view
• Open a UV-editor
• Select “Unwrap Structure …” function and reselect you complete structure
• In the UV-editor select “planar mapping” (tab-key) and select copy afterwards
• In the O2 window select undo to undo the “Unwrap Structure …”
• In the UV-Editor select paste to regain the UV-mapping

Now you should have your structure more or less correct UV-mapped, you only need to move the UV-maps, so they do not overlap.

On more complex structures, use professional modeling software which has good UV tools.