^Maybe cut a wood jig to go on interior, and press seam tape on from the 'flat' side first, then snip/fold the section extending out, and press this into and around the curved section? Failing that, masking tape + silicon sealant gun?
Are you selling them, and so putting tape on it for aesthetic purposes?
If not, I'd trim one seam allowance enough that you can fold the other over it, and then sew through that.
^Maybe cut a wood jig to go on interior, and press seam tape on from the 'flat' side first, then snip/fold the section extending out, and press this into and around the curved section? Failing that, masking tape + silicon sealant gun?