I recently did the exact same thing. No need to build a custom chest. I took an Engel 80 quart chest that was deep enough, tall enough, but too wide, and sliced off 9 inches from the middle with a bandsaw. Resealed the seams with the rest of an open tube of marine adhesive 4000 (because it was there...) and voila! Instant custom icechest/companionway step.
Pricey? yes, but you don't need to start with an Engel chest either. I just needed bang for the buck for size/insulation. And no, cutting it in two has had no decrease in effectiveness. 3 bags of ice (30 lbs) lasted 8 days each when kept in a climate controlled 70*F shaded environment (um, my house)