What choke tubes can you use for this gun?

i have a centurion khan model 20 survival shotgun in 20 gauge. it is a single shot, lately i been taking it with me more than my 12 gauge due to the fact my 12 gauge is more bulky and heavier. but the problem is my 20 gauge has an open choke and i don't know what kind of choke tubes to use. i never used choke tubes because my 12 gauge is a fixed full choke and i've always just had luck at that range but i want to be able to hunt more with the 20 gauge even though it the recoil is about the same as my 12 gauge due to how light it is. so what kind of choke tubes can you use with a centurion khan model 20 survival?