Alcademics

I found a cooler and ice cube tray combination that makes clear 2.5 inch cubes. The tray fits into the cooler at the top perfectly - you don't need to make any special effort to hang it nor do you need a riser to sit it on.

This extra-large size cube fits the Cocktail Kingdom 55mm ice ball maker or you can just use it as a starter cube from which to cut diamonds. 

The equipment for this is:

• The Igloo Mini Playmate Cooler, 4 quart, 6-Can Capacity
• Cocktail Kingdom 2.5 inch square ice cube tray 

You might also want a drip irrigation hole punch to poke holes in the bottom of the ice cube tray. (A larger hole punch would work great too.)

(Note that I also tried purchasing a different 2.5 inch ice cube tray on Amazon but it's not actually 2.5 inches - closer to 2 inches. It's this one.) 

Instructions:

• punch holes in the bottom of the ice cube tray 
• remove the top handle from the Igloo cooler. To do so, you can't just pull off the top. You pop out the disc cover on each side, then there is a screw that you unscrew. This video demo shows this. (Note- apparently you don't need to completely remove the lid - I see in the pictures that you can just open it all the way with the lid still attached. but oh well I did anyway!)
• place the tray into the cooler - it should hang from the top. 
• fill the cooler and the tray all the way up
• put it in the freezer and let it freeze for 2 days or so- until the level of the clear ice is beneath the bottom of the tray
• remove the cooler from the freezer, let the ice slab slide out, and remove the cubes from the tray. I'm impatient so I speed up the process by breaking out the tray from the block with an ice pick.