Forget a big bag on a bike.
I use a 58 cm backpack.( Could be smaller but I take some other stuff too sometimes with two sizes of coils and break down the ctx as well as my lunch and drinks. It has several ways to tighten the gear inside of it with side straps and I can TENSE it up so nothing is slouphing around in the back pack. The Shovel I take is a LONG handle not a short 36 inch, but I strap it to the bike with bungie cords and keep the metal part by the back wheel under the seat. that way I dont have to feel worry about it sticking out the top. The Top of my carbinfiber rods will sometimes stick out of the top side of the back pack depending on how big of loop im using. As long as its secured in the bag and I don't plow through tree branches It's ok.
I like this set up to get me to the site, then I get off the bike and walk draging the shovel. Ive put a ski pole wrist strap to the end of the shovel so I dont have to always grip it and it will just drag if the ground is decent and save my hand grip. Ive also taken some elec tape and make the handle have a bit of a larger smaller ball shape so I can pull it sorta with one hand more easily, hard to explain that. but think of a hammer with a bump in the handle made to fit my grip. it works awesome and have it also on my sand scoop and other tools.
I used to use the carry bag. for me...too floopy and cumbersome. Backpacks are made to sinch tight and this is much better for the ride also for a hike as the shovel works as a walking stick support too.. I also wrap the detector screen in my jacket inside cuz Im a pussy and dont want scratches. But I have hiked biked and hiked into areas a big bag would wear you out on.