Have been thinking about the some of the same.
Currently use a Burant tunnel bag purchased through Polaris. See Pol PN 2882822 (and others). Unlike most you see, it has a shovel "sleeve" on the outside/on top of the main cover. That way I can access the shovel with out actually opening the main part of the bag. The bag itself has moveable dividers to create your own compartments. My probe is then inside the bag (if I need that, I have more important things to worry about than getting snow inside). Works great ! (IMO)
Side Note - I also carry shovel and probe inside my back pack in case I ever get separated from the sled. And then I don't have to remove my pack...... [Avy/air packs don't do much good when sitting on the snow beside me after taking it off to get to my shovel .......
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