As someone who has actually cut foam slabs on several occasions, I recommend a purchase. In my case, if the result was not acceptable, then a purchase would be made. Nothing to lose and everyone agreed in advance.
If you have no carpenter hand working skills, this will be challenging. This process employs a full size hand saw. And you should make a practice cut to see how patient, accurate and persistent you can be.
Warning, Don't expect to measure the final surface against your surface plate. However it was better than i expected. I did use a coarse old (but sharp) saw. The old rule of thumb applies here, the softer the workpiece, the coarser the blade. Also, make long and slow strokes to keep on the lines. Best that the saw starts on a corner, so you can aim the saw against two of your perimeter lines. Another piece of advice, whether drawing a straight line or using a band or power saw, keep shifting your eye focus to the distant point where you want to end the cut.
Procedure: Draw a visible cut line around the perimeter with a good marker that will dry and not bleed later. Stand the slab against a solid surface, which could be a sheet of building board or wall. Double-face removable carpet tape will help to stabilize. You need to stand with the edge of the slab central to your body, so you can maneuver the saw to make full length strokes without impediment.
Let the saw do the cutting of its own weight, don't apply pressure down.
Paul
Edited By Paul Fallert on 31/07/2020 19:23:00