"Biking": I assume you mean bicycle not motorcycle. I have yet to find a satisfactory carrying technique for a DSLR on a bicycle:
- Storing it in a bag on my back takes too long to retrieve when a photo moment presents itself. It's okay for landscapes but not fleeting street scenes and wildlife.
- Strapping it to my chest keeps the camera quickly accessible. I made something similar to OP/TECH USA Stabilizer Strap. It seems too risky in a crash, likely to ruin the camera plus my ribs.
- A side belt holster okay for a tiny camera but a DSLR+lens is too big considering all the hip movement for bicycling.
- A bag mounted to the bike will subject the camera to more jolts than if it's on your back or chest.
I usually leave the DSLR at home and bring my Ricoh GR bicycling. The OP is talking about a 300mm lens, though, so my solution doesn't work for the OP.