You could also try simply cropping to taste and exporting with a width of 1080. That would mean that the height would be less than 608; I don't know what Instagram would do with such images, but you could experiment.
Instagram works best with an image width of exactly 1080 pixels. Height can range from 566 pixels to 1350 pixels.
Do you use Windows or Mac? If Mac, the Preview app has a markup tool that lets you adjust the size of an image, putting borders around it.
Hah - thought I was going senile - couldn't make this work after realising that you weren't talking about the MAC OS Preview, and then downloading Preview Planner for Instagram on to my phone. There was no sign of the option to add frames so I went on the developer's site and it currently says this:
"NOTE: The borders are not available in Preview anymore. We are working on brand new borders. You can still use Preview to plan your feed in advance, use our beautiful filters, and find the best hashtags. If you are looking for borders some alternative apps we recommend are Instasize, Pic Stitch and Photo Collage."
.... so I will try Instasize.