As the most simple basic needs. No sharp edges like if the plastic has a seam sand it down smooth. Run it across your tongue if it feels sharp its not safe for your ass. Also need a way to ensure it doesnt get lost inside so make a lanyard or retainer ring/handle. unless you plan to keep it full length where its too long to go all the way in but when you say plug and not dildo i assume its for longterm wear and not just fucking. Also make sure its clean and whatever way you modify it dont have pieces that can easily fall off inside you.