I have thought the same thing.
A few things to consider:
1) the flaps could be made of either plastic strips or solid card. Flaps in 1;400 are quite thin in comparison to the thickness of the wing. You could make a template for each type - obviously a 747 is going to require some work.
2) If you design the template right, you could attach it to the moulded flaps over the top of the moulded wings. Darken the existing moulded area to represent the flaps dropped and creating a shadow effect
3) You could butcher a DW model easier than most. Diecast cutting is way too ambitious for me.
Either way - good luck with that. Check the ANA precision thread with their 744's with the flaps down etc. It looks good if you get it right.
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