Hey all, messed around in Photoshop and came up with some area of effect templates you can print out and lay on top of your maps to determine spell ranges. Here are some small previews and then the pdf is attached below. Thanks!
First off, cones always have a front width the same as the cone length, and the front of a cone is always flat, not curved.
Xanathar's Guide outlines two ways you can apply AOEs on the battlemap.
The template method, where you basically overlay a template on the grid, and all squares touched by the template are affected. UNLESS the template is a sphere, in which case squares covered 50% or more by the template are affected.
And the Token Method, where you basically get what Matt Colville meant when he called 5e "non-euclidian": 5" squares are 5" in all directions. In this case you put out d6's or other tokens, and keep the length in squares the same, even on the diagonal. In this method, a 30-line pointed diagonally still covers 6 squares, and a 30-foot cone always covers 21 squares.
4E and 5E have the oddity that a Fireball like effect fills a cube, not a sphere.
I've made a number of these templates out of wire. 12 gauge house wire, from the hardware store, works well. The different colored insulation available makes it easy to color code the different templates, and you can lay them down around figures already in place, for lasting effects.
To join the ends of the wire, create an overlap in the insulation by flex-breaking the wire inside the rubber coating, then slicing the insulation half an inch away. Expose half an inch of copper on the other end, add a drop of glue and insert Tab A into Slot B, so it stays as a closed form.
I plot my bends by tracing the pattern on the battle mat, then laying the wire out and following the lines.
Fireballs in 5e only fill a cube if you use the "token method". I revised my post above so it's more correct referencing both methods descriebd in XGE.
If you use the "template method", which may be the default method assumed by the designers, a Fireball is a sphere overlaid on the grid any way you want, and only creatures in a square filled 50% or more by the sphere area are affected.
So a 5e Fireball with template method, centered on a grid intersection, affects this area: