For those of you out there who have built one of these, I could only find 1.5"x1.5" 1/8" thick angle iron. I am wondering if this is going to be strong enough to support the cabinet with the media inside. The directions call for using 1/4" angle iron which seems to be very beefy for this cabinet...

So, will the 1/8" thick be sufficient?

I assume you building the wooden plan they market ? IMO the .125 =1/8 in angle would be
sufficient.
It's at the bottom where the legs extend ther would be any moments.. you can overcome this by a few flat strap bracing. Xing the back and a digional on each side..
Kind of like Belt and Suspenders notion..
do that and it will be STRONG LIKE BULL

Okay good. All Home Depot carried was 1/8" agle iron. I guess I could go to the 2" instead on the 1.5" that would also beef things up a bit.
MAN, that stuff is pricey for a 6ft length too.

So far I'm in this cabinet for almost $320 and I still don't have any filters hooked up. That's gonna be another $100 for that too. This is getting expensive, but hopefully it'll be worth it in the end.

I used 1.5 inch x 1.5 inch angle iron, no thicker than one eighth. It is adequate. I did not brace the legs diagonally. It is sufficiently strong if you brace the bottom of each leg with a piece of angle or flat iron, joined to the other leg. I joined the three legs on each side and at the back. Do not brace the front on the floor because it will interefere with the foot pedal and your sliding out your pail of media when you empty the hopper. Remember, you only need one bag of media inside the cabinet, which doesn't weigh that much. The rest of the weight is the plywood. You will be happy with the cabinet once it is finished, and wonder how you got by without one all these years. Best tool I ever built.