The easiest thing to do is go down to your local garen centre and just buy one, they probably don’t cost much but if you still fancy making one, here is a youtube video i found showing you haw to make it.
Basically use an old glass globe from a an old lamp or buy one lamp costing £30, throw away the lamp keeping the glass glob shade and make a concrete globe worth £15. Makes sense, also remember you will have to buy the ready mixed concrete as well. All very logical.
Fill the globe with concrete, allow three days or more for the concrete to set and then use a hammer to break the glass, trying not to cut your hand off with the broken glass and ending up in hospital will 20 stitches.
I can’t say I would want to make a concrete globe using glass as a mould.