An erect penis is recommended.
If you’re low on supply then a table tennis ball and piece of string or shoelace actually works remarkably well. there’s even a video:
You can use the same technique with a squash ball to make a rubber version if that’s your thang.
For a denial slut your freshly soaked panties shoved in your mouth followed by a strip of duct tape (stick it on some material first a few times to reduce how sticky it is) is a quick, easy and tasty option.
Always make sure you can breath properly and are able to remove it if you need to. Remember not to answer the phone while gagged, it will confuse the caller no end.
Then again you can buy a proper ball gag from Lovehoney from about £5 so… if you can that might be a better option.
If gags are your thing then make sure to check out: