Jaxx

A variation on what has already been suggested is to make a 'face on' version of the "CHANGE COVER ART" layer as a separate Affinity Photo document, using vectors for the zigzag pattern, corner seal, & so on, something like this Killer Cover.afphoto document I (not too carefully) made. Embed that file in the template document using File menu > Place, & then use the live perspective filter to adjust it to the same shape as the "CHANGE COVER ART" layer:
  
 
Because the replacement cover is a layered, embedded .afphoto file, double-clicking on it in the canvas will open it in a new document window, where the vectors & text can be edited or scaled, like in this particularly ugly example:

See if this recent thread is any help
 
 

If you absolutely have to have the alpha channel for whatever GUI you are working with to display the graphic, & you only have the Affinity apps to work with, you might try forcing the creation of the alpha channel, for example by using the corner tool to change one corner of the rectangle to have a radius of at least 0.5 px. (Anything less & the app will rasterize it to a zero radius.) This should be almost undetectable in the GUI because it affects only one pixel.
 
You can also try rasterizing the graphic (which exporting to PNG will do anyway) & using the Erase Brush set to 1 px width & less than 100% opacity (I tried 65% & it works) to partially erase a single corner pixel, which should be even less detectable in the GUI.

I use LR, but I am so ready to cut Adobe loose!!!
 
Just waiting on the Affinity DAM to pull the trigger...

Hi Jaxx,
Welcome to Affinity Forums
Affinity doesn't include the alpha channel if the PNG doesn't have any transparent parts (and the background is set to transparent).
I will fill an improvement request for this.

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Thanks for everybody's input on this one :) Just to reiterate, version 2 will be a new version, not an update to version 1 - all updates are free and this means within the version you have purchased - this is, as has been pointed out, 100% standard practice for software for the last 20+ years. With regards to upgrade pricing for version 2, well, who knows?... the reality is that our focus is utterly on version 1 and making it into the product we all want it to be. We have not written a single line towards version 2 at this point and I don't imagine that will change for a long time. We set out with the intention of creating fully-featured, professional level software that was fast and fun to use, and we set a price that we thought would be fair so that we could sell it to lots of people and they would all be happy - we intend to keep adding features until it reaches a mature level that we are happy with, then we will start to write version 2...
 
If we don't put enough into version 2 to make it an attractive purchase at whatever price we may set, then that is our problem - and we will pay the penalty for it, but that's a long time away before we need to even think about it... ;)
 
Incidentally, perhaps somewhat differently to other software companies, the people involved in writing this software actually use it and enjoy it - and we've also all bought it with our own money as soon as it went on sale. We want the same experience as our customers and we have the same expectations. If we did something that wasn't in the customer's best interests then we'd also be letting ourselves down - and I certainly don't want that...