I could not find a plugin to create Polaroid SX-70 style borders, so I created one myself. By default it adds a slight shadow around the opening and also a slight tone to the white border, making it more "natural". The drop shadow is optional. It can also add a caption to the lower, wider white border.
The script can be easily adjusted to make regular white borders.
To make it work for all picture sizes, the border width etc is specified in % of the picture width.
The picture format is not changed, so to create a regular Polaroid look, the picture has to be rectangular.
The picture itself is not modified. To get the right look, the Lomo plugin is highly recommended, with suitable tuning of parameters.