The question of ammunition is VERY important. No bullets no fun.
I build my mortars around commonly available ammo - (why do beer cans come to mind first?) such as 4" pvc pipe (4.5" OD), soda pop bottles (16-20-24 oz size - Coke and Pepsi are the SAME caliber), beer cans, golf-balls.
Some folks do D, C and AA battery calibers.
Then consider copper tubing - 3/4" copper tubing turns out to a bore of about 0.90, half inch copper tubing will be 0.625 which also works well as 20 ga. slugs. Also are the connectors for the tubing - you don't have to cut sections.
And of course there are the standard sizes of round ball moulds to consider - 12 ga (0.73), 0.69, 0.58, 0.50 (0.49) and smaller.
Fillers for cylindrical containers can be most anything. Sand, cement, water (with reduced loads) and silicone potting compound are a few I've done.
There is one gentleman here who mentioned doing one in 0.177 caliber!
And one that I forgot - the Fuji (Kodak ones have a lip) plasitic film canister.
Obviously there are more, these are some that I've tried.