You can buy a commercial rifle rest, stick, or bipod for $20 up to $100.
This one is very easy to make and costs very little; way less than $10.
It is versatile, adjusting to many situations and can also be used as a walking stick.
There are similar designs online, but you will find this one the easiest to make.
It is also very reliable and easy to repair in the field.

Step 1: Parts

1) Two 5 foot (1.5 meter) garden stakes, rubberized steel, 1/2" (12.5mm)
2) 3 O-Ring hose washers
3) 24" of rubber tubing (Rainbird SWGP2 2' sprinkler hose)
4) 3-3.5" Cable ties

All of these items are available at Home Depot, Lowes, and most nurseries.

The 2' Rain Bird tubing is 1/2" (12.5mm) ID. You can substitute lots of different types of 1/2" ID tubing including clear nylon tubing. The Rain Bird stuff is easy to work with and has good friction and feels good when using the rest as a walking stick. It comes in a 2 foot length which is all you will need.

Try to get stakes that are closer to 1/2" diameter than 7/16". They are stronger and work better.

Step 2: Cut Parts

Start by cutting the two feet of tubing to two 7 inch and two 5 inch pieces.
You can also cut them to different lengths, such as 4 pieces of 6".
I just found that I liked the top pieces to be a little longer when using the bipod as a walking stick.
The length of the two bottom pieces can really be much shorter, like an inch.
The top piece should be about as long where the "hinge" made from the o-rings should be. It is not very critical. The height adjustment is mostly by how far apart you spread the two legs. A tall person may prefer the hinge to be a bit higher.

Step 3: Assemble

1) Take a hose washer O-ring and place it over the pointed one 5 foot stake.
2) Twist it 180 degrees and push it over the pointed end of the other stake. The pointed end is easier to get the twisted O-ring started.
3) Push it up an inch or two, and repeat the process with another O-ring. You can try twisting two different ways to make it look neater.
4) Push both O-rings to the top, flat end, of the two stakes, about 7 inches from the top.
5) Slide the two 7 inch pieces of tubing over the top of both stakes.
6) Slide the two 5 inch pieces of tubing from the bottom up to the two O-rings.
7) Leave about 1/2 inch of stake exposed at the top, and tighten a cable tie above both top pieces of tubing. Cut off excess tie.
8) Push the pieces of tubing, two O-rings, and bottom tubing sections up against the top cable ties.
9) Apply another pair of cable ties below the two bottom pieces of tubing. Trim ties.

Step 4: Final Notes

Take it outside and try it out.
Spread the legs to where the height you like the best is obtained, or to whatever the terrain dictates.

Use the third hose washer O-ring at the pointed end to lock the stakes together when using the rest as a walking stick, and to have a spare O-ring in the field. I have yet to break an O-ring, though. Other designs using screws weaken the stake and are not as easy to fix in the field.

You could fashion a sling using cable ties and nylon straps, for instance.
I like just using it as a walking stick. Nice and light, but strong.

Nothing better than saving a buck making something that you need, and that works well.