Hi linzykins,
As long as you aren’t making horizontal cuts to “slit” the foam into a different thickness which would generally need specialized equipment that you would normally find at a mattress manufacturer, a foam fabricator or supplier, or perhaps a fabric or upholstery store … it’s not difficult to cut foam to size using an electric knife and all you need is an electric carving knife and a straight edge for a guide and just go carefully and slowly and move the knife in one direction (not a sawing motion). There are some great instructions in post #3 here and you can also see some pictures here of a home setup that was used to make some more complex cuts using an electric knife.
There are also many manufacturers that sell component latex mattresses that will do side by side splits in queen or king sizes (including many of the members of this site that sell latex mattresses online that are listed in post #21 here) that allow you to choose different firmness options on each side of a mattress. There is also more about split layering in post #21 here. Some of them sell zoned latex mattresses as well.
If you are attracted to the idea of designing and building your own DIY mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
If you decide to take on the challenge then I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) as a reference point and try and “match” them as closely as possible or use a “bottom up” approach (see post #2 here).
Assuming that you decide to pull the trigger on the Spindle mattress … congratulations on your new mattress 
You are certainly making a great quality/value choice and I hope you have the chance to share your comments and feedback when you receive it.
Phoenix