Has anyone tried building inert practice rifle grenades from hardware store ingredients?

Plethora of videos showing people having fun launching these from their AK variants, but when you can find some for sale, they're pricey, way over priced for what they should be, for no more than what they are.

I enjoy fixing, designing and building, so for me it would be as enjoyable in the construction as would be in launching.

I'm looking at having fun through an enjoyable activity, having absolutely no desire whatsoever for historical accuracy nor re-enacting, so whatever I build could look like it came off of a Star Wars/Scifi type movie, long as it was cheap and easy to build, reliable and safe to use.

We are not talking rocket science here, just basic building materials, metal tubing, plastic and rubber.