Something that isn't permanently attached to the rifle, something you carry on you or in your pack.

Russ

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"Out here, 'due process' is a bullet!" -- John Wayne, "The Green Berets"

Russel, the Harris and Snipe pod bipods aren't permanently attached to your rifle. The Harris takes about a minute to attach and the Snipe Pod takes about 10 seconds and both can/could be stored in your pack while not in use, in fact that is just what I usually do.
On top of that, the Harris doesn't require any special attachments, it clamps onto the front sling swivel stud and the sling swivel attaches to the bipod so sling carry is possible. - Sheister

I'll agree with Sheister that most bipods aren't permanent, but I get the jist og your question. Sturdiest I could hope for without a Harris is a perfectly shaped tree or stump with my backpack on top of it.

Shooting sticks.Work great.

my personal bipod....I took two pieces of wood measuring about 1-1/2" X 3/4" and 24" long. put a bolt with wingnut through both at about 4" from one end (forms an X with a small top, big bottom). stuck a piece ofsoft cloth in the top V to prevent scratching. works great, inexpensive, adjustable. this is for the seated position. if you shoot prone, make one smaller. this way you do not have to hold on to the bipod while shooting.

ski poles works great, can use a plastic pipe for extention

Steady Stix II or Safari Stix collaspeable. These are two different sizes for if you will use them standing, kneeling or sitting. They fit into a belt pouch and are about 14 inches long collasped, weight next to nothing, Cabela's has them in their catalog $35-$57. I use the Steady Stix II or a Leki Treking Stick with a gun rest attached.

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Handgun Hunter
LOVE THOSE .41s'

I have used a branch from a tree. It could be a branch from anywhere. It would serve as a walking stick most of the time. I don't have a problem leaning on a tree or going prone and using my back pack, or a rock. The point is almost anything can be used as a rifle rest in a pinch. For me to take an off-hand shot there MUST BE NO OTHER OPTION and the animal must be within MY point blank off-hand range. For a deer sized animal that's less than 50 yards maximum for me. That's because I stink at shooting off-hand.

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From my cold, dead hands!
Thanks Chuck!

Steady Stix or any cross sticks are my choice. I usually have them tied to the outside of my pack for easy access. Have made some of my best shots accross these sticks. I like them much better than anything that fastens to the rifle.