Ok, vet wrap. It comes in 4 inch wide by 5 yards long roll. Thats enough to make 3-4 bras. My roll was $3.59. It even comes in different colors. I wouldn't use red though because other chickens might peck at it thinking it's blood. Its great stuff cuz it sticks to itself like velcro but won't stick to anything else including feathers. I've made 2 bras. The first one I made was totally made from vet wrap, no sewing involved. First I carefully unrolled enough to fold into a double thickness, approximately 10 inches. Before folding I put a smaller (about 2in x 3in) rectangle of cloth from a rag on one side so once I folded it over the cloth was sandwiched inside. This may not be needed, but it was my first attempt. Keep in mind once it gets pressed tightly you generally can't re-press it so be gentle until everything is where you want it. Ok so now you should have the rectangular chest piece. Now cut 2 strips about 18 inches long. I'll admit I never measured anything but that's close. Now lay one of the short sides of the rectangle onto the middle of one strip only going in halfway, then fold the other half of the strip over the top. Do the same on the other side. Tadaaa. Bra! I did discover quickly that you cannot pull a knot tight with vet wrap. You need to make your knot very close to where it will stay. Alternatively pull the ends together but overlap to the tightness point you need and squeeze them together. I find that putting the left top strap at the neck together with the right bottom strap from under the wing, and vice versa on the other side, makes the straps crisscrossed on their back and is more comfortable and stable. Whether you position it with the rectangle vertically or horizontally depends on the chicken. Try one way first and when you need to replace that bra switch it up.
That's the basics. Pretty easy really. Hubby helps me apply it by holding the bird while I strap it in place. If you have a calm bird you MIGHT be able to do it alone. I did make a second slightly different but only just started using it so I can't attest to it being any better that this design. On the second one I actually sewed a piece of felt to the rectangle so as to make a soft side against skin. Actually the wrap isn't very rough anyway so that may be overkill. Use the K.I.S.S method. (Keep It Simple Stupid). Good luck and let me know if you don't understand something.
Heres a picture of that very first bra that I made. It's used and pretty beat up but i think you get the idea.
And here's my Roopecca wearing the last one I made.