Great idea for lining the bumper insides using the coroplast. For my bumper caps, I just drilled a hole thru the top and bottom of the bumper and the cap.
Then I took a piece of coathanger, cut it longer than the height of the bumper and put a bend in it.
Put the wire thru the bumper and cap, with the bend facing outwards. Never lost one yet.
For the flagpole mount, I used self tapping screws to attach a pipe repair clamp to the trailer frame. I got lazy and used JB Weld to hold nuts onto the ends of the clamp bolts so the wingnuts wouldn't unscrew and get lost.
Slip my flagpole into the clamp and tighten the wingnuts (that's not my flagpole...it's a shovel handle!). Isn't going anywhere. Last thing to do is paint it black.
Rick