Open a new image e.g. 400x400.
Fill it white
Add a second layer and fill it with a contrasted color (here red)
Go to "Layer","Load/Save Mask","Load mask from disk"
Choose a one sided mask
Highlight the layer called "Mask" in the layer group. Right click and select "Duplicate".
Go to "Image","Mirror" (or use the "Ctrl+M" shortcut :) )
Duplicate the mask layer again.
Select the deform tool.
Rotate the mask so the border is on top (or at the bottom).
Duplicate the newly positionned layer again and flip it using "Ctrl+I" (or "Image","Flip" menu).
Then go to "Layer","Merge","Merge group".
You now have a frame mask instead of just a side mask and you can save it to reuse it :)
Some additionnal tricks
Resize
When you use a mask, you can resize it as any other image to fit your needs using the deform tool without altering the base image.
sample
Colors
By highlighting the raster layer under the mask you can change its color ("raster 2" on the example)
sample
Effects
I just wanted to show you that you can work with the mask just as with any other layer. It means that you can apply any of the effects of the PSP "Effects" menu on the mask layer or on the raster layer before merging the layer, which gives you thousands of possibilities. Here are two (only :) )