The guides are better than a stem but a stem works in a pinch. I have that guide and have used a few others and it’s ok as they go. I’m a bit far from the makerspace (Stratford) but if you can wait a bit I’m sure we could arrange something if you wanted to use it or help.
Take it slow as you can still go wonky even with the right guide and blade. It doesn’t take long to do with a hacksaw so I wouldn’t bother with anything electrical.
If using a guide, make sure it is secure but not clamped too tight as this can sometimes make the cut not square depending on the style of guide.
If cutting carbon, ideally use a carbon hacksaw blade, but a very fine toothed hacksaw blade will work. If cutting carbon be extra careful about dust, it’s super nasty and can be quite hard to clean up.
Importantly make sure you cut your stem slightly short, as you need the headset cap to compress the stem, spacers, frame and crown race to the fork. Front end will be quite wobbly and potentially dangerous if not. It’s also generally recommended a max of 4cm if spacers below the stem and 0.5cm above but there’s lots of deviation from this.