90% of my riding is enduro racing, in order to get anything out of a back pack I would be removing my gloves, helmet, neck brace, and chest protector. At many checkpoints, tired as hell, no way I would want to deal with that. You are always having to inch forward as the earlier rows leave, etc. I keep my hydro-gels, pain pills, snuff, KTM multi tool, Co2 a few Zip ties, a tiny screwdriver for clicker adjustments (put a piece of rubber hose over the tip for safety), maybe a set of glove liners or an extra tinted goggle lens for when the sun comes out.
For this I use a $7-$15 fanny pack that you can find at any Walmart, Academy, etc. They work perfectly and I have a small size and a little bigger size depending on the event. Been using the same one for several seasons, I think I paid $6.99 for it. I keep the bag in front of me, that way it kinds of rests on the tank and doesn't get in the way of anything. I can grab anything i need out of it very quickly.
Going cheap is just an idea...the Kelly Enduro bum pack pictured earlier is $160....lol I'm out!
Long distance exploration with more tools would be a different story but this setup works for racing and woods practice.