Just ran across this web site for commercial covert antennas http://www.sti-co.com/antenna-products/covert-antenna/hidden-antennas/vhf-bumper -- this one is a bumper mount configuration. Note there are TWO antennas, in diagonally opposite corners of the vehicle. If you are using repeaters (as opposed to simplex) this might be a viable option. The company has many other hidden and covert antennas. Good luck!

Just ran across this web site for commercial covert antennas https://sti-co.com/project/covert-bumper-antenna-2/ -- this one is a bumper mount configuration. Note there are TWO antennas, in diagonally opposite corners of the vehicle. If you are using repeaters (as opposed to simplex) this might be a viable option. The company has many other hidden and covert antennas. Good luck!