怎樣做一個補溫瓶？

If you just want to make a cheap thermos flask with the least material, try to make use of waste plactic bags, like those supplied by super markets or departmental stores for distribution to customers for carrying commodities they bought.
Tear these bags into sheets and wrap a cup by these sheets, including th cover (the lid) and the bottom. You may tie the sheets onto the cup using rubber bands. If you have wrapped about 3~5 mm layer thick of pastic sheets onto the cup, the insulation property would be quite acceptable.