On October 2020, Redbubble released fitted masks for sale. This is the template Redbubble released for fitted masks. Right there on the image, it tells us that the right dimensions for this product are 2716 x 1972px.
However, when you download the file and look at its dimensions, they are different. The file itself is 2048 x 1487px. So if you create art on the template, it will be too small for upload.
Additionally, the file was in Greyscale mode. It would make much more sense to be in RGB mode, as most other Redbubble templates are.
I fixed the Redbubble template by resizing it and converting it to RGB color mode. Download it below.