So here I was racking my brain on where to get 5mm posts. I googled, called hobby shops, but the largest Plastistruct styrene posts was 4mm. So I got out my calipers and went shopping, measuring products off the shelf. In Walmart's soda aisle I came upon Silly Straws. They're exactly 5mm and only a $1.50 a package. These are solid, really strong straws made of ABS/acrylic or something similar that won't bend or squish.
To cut them roll the straw on the table while rolling the edge of your knife along the top the same way you'd cut a Styrene rod, then snap at the score to make a perfect break. They're hollow and octagonal inside so you can insert a screw/post into the weapon for added strength. I recommend prepping them with Loctite's Plastic 2-part Superglue system. Just drill a hole in the weapon you want to add the post to, use the primer, then glue! Now anything can fit into a mech-tech port or hand, our search for 5mm posts is over!