Because the male clips of the buckles on my otherwise highly-resilient Highlander rucksacks broke through fatigue stress, I've made copies of the clips with the brand name 'Rock-Lockster' printed on them.
I've redesigned the clip for the 1-inch straps so far, I might work on one for the 2-inch-strap buckle that goes around a wearer's waist, but on that part both sides of the clip can be removed without undoing any stitching, so there may already be a buckle on thingiverse that can suit that purpose.

Current results:
Clip 1.0 was very stiff in ABS(I was trying to make the base of the male teeth thicker to prolong service life, but kinda over-did it), so I redesigned it slightly slimmer in version 1.1; I don't yet know how they perform in PLA, maybe someone out there with one of these bags could test it before I do (once I fix my hot-end again, more about that on my blog soon). Version 1.1 works just fine in ABS, though the joint in question shows slight bruising on the outside, where the clips broke before.
(Versions 1.0 and 1.1 are shown in a photo on the left and right respectively, next to a penny for scale)

As for the original clips, I can't tell whether they are made from polyethylene or polyamide, but they are injection-moulded very smoothly. The D-shaped section in question, about 2.5mm thick from curve-top to flat and 5mm wide along the flat, broke after about 5-8 years of deflecting about 20-30 degrees and back again daily.

Print a buckle clip.
Test it out in the corresponding buckle.
If it works fine, remove the old broken clip from its strap and replace it with your new one.
Straps with the end folded over and stitched may be difficult to fit through the slots (even more so with the original design), if you find this to be the case then try push/pulling those folded ends through diagonally.