This was my first attempt at building a bumper.
Took my off-cut square tubing and slapped them together, doesn't look good but it was just a template to give me the basic idea. Please don't judge from that.
Made a few cardboard cut-outs and stuck it to the template, took the cardboard template and put it flat on the 4mm sheet. Measured it exactly to ensure that both sides are exactly the same. Cut out the template, Cut lines into the cut-out where the bends must be, started bending and this is how it turned out.