I got a drybag daypack from REI 25 years ago, it has an oval bottom.
and was found to be big enough for my mandolin case..

You.. Could add a sternum strap.. between the shoulder straps ..

I think it was actually made by Seattle Sports.

Need to carry More ? Portage Pack. I got one from Cascade Designs, 'Pro' pack.
works in my Bike Trailer .. once secured in the trailer , If needed , I can wear the trailer too.


I own a Timbuk2 messenger bag.. it included a cross strap., a TriGlide on a loop was included
when the side seam was sewn.. the other end is a sliding loop around the main shoulder strap.