NEWS !!!
Almost becoming crazy to deal with the chinese factories.

History: we started with 2 factories that confirmed from the beginning "we can do it" (but that is typical chinese).
First factory, we ordered 3 filters: ND8-ND16-ND32 Conclusions: too much weight and the ND16 and ND32 bad quality.
Second factory: I suppose the filter quality would be OK, but, they have problems with the frame thread. They need to buy a german machine to cut the right screwing. Complicated.
All good things you need a 3x exercise…..
Now 3th factory, "highly" specialized in filters and coatings, all Japanese high precision machines. We talked a lot and the CEO likes to make special stuff, not ordinary filters as we find everywhere as Altura, Neewer, General Brand …. (all coming from the same chinese factories).
We sent him our dij ND4 that arrived broken,

. Frame and thread for them is no problem and weight should be not neither. Firstly they tried to polish the glass to achieve the right weight, but that would become too expensive. Now they find, after analyzing the DJI one, the right solution. Glass of 0.5mm and apply the right coatings with their high tech Japanese machines. As he said the ND-grade of most chinese filters is not very precise (I agree), so not what we want ! He is excited to produce a high end product !
Listening to your requirements, we think to sell sets of ND8-16-32 (-3,-4-5 stops) with the possibility to add ND4 (-2 dij for those, as me, who haven broken) and/or ND64 (-6). Today we checked the price for sending to the US or Europe by TNT : 28 USD. This high tech set (3 filters) will be +/- 45 USD (not finished to deal, but we need to order big quantity > 300). Extra filter will be 16 USD, without change in transport.
=== shortly we will receive a sample ! === be patient, I almost gave it up, but my chinese wife luckily found this factory.
By the way, in China (always much cheaper), the price of a DJI-set is 249 RMB (40.2 USD) excl. transport for the original set (clear and ND4) at their store.
Regards, Renaat