Hi, check out the free design of mask we made available at www.maspire.org. Free to use for non-commercial purpose. The shape should get you started.
We've donated low thousands of similar masks to LE in our area (slightly different design allowing for efficient cutting and sewing).
In our experience the side folded masks make for poor nose seals. You absolutely need a stiff insert on the nose for that (electrical copper cable is fine).
Fabric: cotton ca. 180g/sq.m. is fine. We use vlieseline as the inner layer/filter. These are breathable and can be washed up to 90°C.

Their quality is fine.
However we mixed the 1" SRBs with ITW ones a few years ago and ended up with broken female buckles. I don't remember whether the females were GSI or ITW but there was a small thickness difference which puts a strain on the female SRB.
As long as you don't mix they will work well.

Our experience has been different with ROC buckles.

We now have a few hundred carriers out with them. Some in daily LE use and some went to harsh places (jungle, desert, maritime).
A few have broken their locking latches (from pulling too hard on them) but even then the buckles still function. You could break two of the four lugs (ROC80) and the buckles remain operational. The Tubes won't work if you break their locking latch and replacement buckles are hard to find (the French Army sends the whole NFM carrier for repair when that happens).
Having 2 wide slots makes for an easier replacement compared to 3 narrow slots - we cut ROC buckles to offer a repair version until 2M has them available.
The alignment can be a PITA but the goal is to prevent the relative rotation of the two pieces from straining the locking lug.
Unless you have a non-elastic cummerbund there's just a proper gesture to learn to fasten them.

Besides, it's not like we have a lot of options in Europe. The Nat Molding one could be a better design but we can't buy them so who cares.

At Impall.pl

Excellent news and thanks for sharing.
Do you laminate MC fabrics with solid color Cordura or is it two-sided MC?

NFM doesn't sell Tubes to anyone AFAIK.
You can also use the new Duraflex buckles that were designed for Lindnerhof in the first place.
However the ROC buckles are better noted by users when compared to Tubes.

The Foliage and Tan shades from ITW changed quite a bit a few years back - your buckles are probably the old shade.

3,5mm as standard and 5mm for sewing folded edges on fabric with a tolerable shortening of the finished length.