This mount allows you to quickly and securely attach your camera to the shoulder strap of your backpack. Using this mount you can access your camera at any time during activities such as hiking and never miss a moment. These types of mounts already exist (see Peak Design Capture Clip) but often cost upwards of $70 which is a lot for amateur photographers and hikers. I thus took the challenge to design a similar mount myself and I'm quite pleased with the results.

Main features:

sturdy mount allows for heavy DSLRs (Canon EOS 77D + 55-250mm zoom lens)
camera is easy to attach and detach
locking mechanism prevents camera from slipping out
metal plate in hotplate for increased strength
attaches to any strap up to 56mm in width and 15mm in thickness
Additional hardware for mount (non printable parts):

1/4' camera/tripod screw (exact dimensions: overall length 21mm, thread length 12mm, shaft 5mm, head 4mm high 12mm diameter)
2 M6 bolts (about 30mm long, flush head with square neck would be ideal)
2 M6 nuts
2 washers (max. outer diameter = 16mm)
sheet metal about 1,5 mm thick for reinforcement plate in the hotplate (see pictures) and the rear mounting plate (can be printed too)
optional: rubber to prevent sliding of the mount on the backpack strap and improve grip of the hotplate on the camera body
Additional hardware for locking mechanism:

steel rod or sturdy wire (diameter about 3mm) for the locking pin (see pictures)
small spring from a ball point pen
EDIT: some instructions for assembly:

hotplate: the metal insert/plate is not strictly required but it saves your camera from falling out of the mount if the hotplate were to break. To assemble the hotplate make a sandwich: start with "Hotplate_Bottom.stl" then put some double sided tape on it, add the metal plate (shaped like the upper Hotplate part), double sided tape, "Hotplate_Top.stl", double sided tape, rubber pad. Finally insert the 1/4' tripod screw. Total thickness of the lower part (which has to slide into the mount) should not exceed 8.88mm.
locking mechanism: the main "Mount_top_v1.1.stl" part has three critical openings: on the left where the push button will be located a square hole that goes all the way through the part to the right. On the right a larger slot designed to insert the metal rod during assembly and a hole inside the mount where the rod will protrude and hold the camera securely in place. To shape the rod print out "Camera_Mount_A4_1.1Scale.pdf" on A4 paper at 100% and try to match the dimensions of the green rod in the drawing exactly. If you do that it should fit in perfectly. Lastly slide the small spring over the outer end of the rod and glue/press on the "Button_Lock.stl".
screw plate ("Mount_Bottom_v1.1.stl"): use the printed part as a stencil to cut an identically shaped piece of metal about 1.2mm in thickness. For the square neck bolts I used: drill holes into the metal slightly smaller than the square neck of the crew then gently hammer the screws into the metal. Make sure you guide the screw through "Mount_Bottom_v1.1.stl" before you hammer them into the metal. In the end the screws/bolts should be nice and parallel, sitting snuggly in the metal so they can't spin freely when you tighten the nuts later.
nut covers: Gently hammer the M6 nuts into "Nut_Cover.stl", if it's too tight (it should be quite tight) try printing the covers slighly larger. Ideally there should be no need for any glue but feel free to use some if you have to.
rubber pads: get some 2mm thick rubber (or similar material) and cut it to size. Attach it on the back of "Mount_top_v1.1.stl" and on the metal plate of the screw plate with double sided tape.
Printing info:
Printed on Prusa i3 MK3S, PLA 0.2mm layers, 4 perimeters for strength, main mount was printed standing upright to improve strenght of hotplate receptacle.

CAUTION: Use at your own risk. Secure camera with short strap for safety in case the mount would break. I am not responsible for any damage to your gear. Have fun and use common sense ;)

This is my own creation and the model can also be found on Thingiverse thing:3771579

Print instructions

Printing info:
Printed on Prusa i3 MK3S, PLA 0.2mm layers, 4 perimeters for strength, main mount was printed standing upright to improve strenght of hotplate receptacle.