It would be absolutely fantastic if somebody could print one off and then test it's strength ?.
If it was made of a solid construction, rather that rear honey comb hatching design on the rear, it has too be MUCH stronger than the original OEM part surely ?

?.
I don't have either the know how or equipment to give it a go TBH - Sorry.
There must be a drink in this here, for some kind member in producing these ?.
The OEM price for a new door are not that over spiteful, but the labour charges to replace the flap are.
Especially when you consider this could well be a annual expense.
You would expect this to be covered by the warranty in the first 12 months regardless.
Trim items are normally covered for 12 months at least.
If enough of these where replaced under warranty and the labour costs became a "red flag" to MG then the quality / strength of this item would soon get improved I am sure.