Whilst we don't have a border outline option at this stage, we do have a short-term fix you can use. Here are the steps:
When logged in, click on this link to access the cutting outline: Cutting outline template (NOTE: this will overwrite any design you are currently working on)
Choose your relevant product from the first tab
Toggle on 'Same Design on each' in the second tab (if applicable)
Add images & design
Then, click on the double up arrow on the border object on RHS, to move above your layer
Do you need to design on each topper, but still want the outline? Then choose the relevant template from the list here: https://topperoo.com/templates/cutting-outlines