Mike from Robopromo.com here. Helping teams make professional-looking bumpers is what we do. I may have some resources that are helpful for you. We recommend using iron-on vinyl numbers (which won't come off) and this year we are offering custom-cut numbers to enhance the look of your unique robot.
For attaching bumpers I would recommend using brackets, bolts, and wing nuts. Most teams find this to be a good method. We offer a hardware kit for that on our website also. For a visual on how to use them, reference the end of our "Bumper Fabric Upholstery" video tutorial.
Here is the link to the video:
http://www.robopromo.com/category_s/1828.htm
Iron-On Numbers:
http://www.robopromo.com/category_s/1820.htm
Bumper Hardware Kit:
http://www.robopromo.com/product_p/rp-2013.htm