My valve trombone case came without a strap and I had a really hard time finding anything online by just searching for "long shoulder strap".
So I was happy when I happened upon this when I was looking for other instrument bags here.
It can be extended really far, and the padded "pillow" has velcro straps that can be undone so the pillow can be moved anywhere on the strap (the metal brace that allows you to adjust the length of the strap often gets in the way on other straps). Instead of clips or carabiners to attach the strap to the bag, this strap simply wants you to thread the ends through the loops and then back and into a brace (similar to the adjusting brace) that holds it in place. It only gets tighter the more force is exerted on the strap, so it seems very sturdy! I like this design.