First get a plastcc or wood barrel (ex. candy barrel, popcorn barrel). Second spray paint it brown. Next print out a message from your computer like warning or caution. Glue or tape it to the front of the barrel. Now download the jpeg. pic. from below by right clicking it and choosing save as. Adjust the image to the size you need in a program like paint brush, print it out and stick that on the barrel too. There is also  a picture of the finished prop below.
Here is the jpg you need to download.
The finished prop.