^ You'd be better off having a leather craftsman make you a couple of extensions with buckles. Don't hack up straps off another bag if you can avoid it. Bridle leather is readily available for saddlery, belts, etc. It may not match perfectly but will look better in the end.
I ordered bridle for my DIY shoulder straps recently.
Thanks for the suggestion, I considered that idea but personally, I wouldn't be able to live with it unless the leathers matched correctly.
Do you happen to know what kind of bridle ladder is used by Filson?