There are some very good low temperature thermoplastics that might fit the bill. These are often used by occupational therapists to make simple splints. It comes in sheets, rod or pellets. By dipping the material in warm water (some can be worked just by the heat of bare hands), the plastic can be formed to whatever shape you want. It can be heated/shaped/reheated again and again.
In it’s hardened state, it will more than withstand the pressure you describe without issue.
I see… I don’t know of any plastics that can be cold formed without the use of a hardening agent. Some reactive metals have great cold forming properties, like niobium. It’s relatively inexpensive, highly corrosion resistant and extremely maliable… but it is a metal, sorry… hope you find what you need!
not high-tech, and not _especially_light, but highly malleable … copper
depending upon length and cross-section dimension of course;
i.e. you will probably be able to bend a .062" dia. x 1.0" piece of round stock with your fingers. A .125" dia. x 1.0" piece of round stock; probably going to need to use pliers.
some dimensional information would help out the decision making process a great deal.