Take a nice picture of the weapon you want to wear at an anti "realist weapon" convention. Here is the FN P90. The image of both sides of the weapon were blown up to full size and glued to foam board (aka presentation boards)

Cut them apart to make them easier to work with.

Do a rough cut to get it out of the larger piece of foam board.

Using a ruler and an x-cto knife I trimed it to the edge of the images


It should look like this

Put both sides together and glue, now you have a convention safe weapon!