So big brains behind HF Series 3 made the boxes slightly (1/8") larger than Series 2 at least for the 26" boxes (now "27"). This means you can't drop a series 3 on top of a series 2 cart... The other way around works for the 2 people out there for whom that'd ever apply.
At any rate... Craftsman 2000 series 26" top boxes fit Series 2 carts with about an 1/8" to spare. They are 26 1/16" wide. Series 1000 should as well.
You could also use a wooden panel as a spacer to get a series 3 upper box over the raised edges of the lower box. If you're worried about the top box sliding off, the spacer can be bolted to the bottom of the upper box so that the raised edges still function as a barrier to capture the upper assembly.
I thought about that, but am lazy plus craftsman was cheaper. And maybe a pinch of petty "screw you harbor freight" ; )