Finally got the bastard finished.
I bought some more steel from Springvale Steel and some castors from Richmond Wheels and Castors, bought it home and got to it.
I had some 32×12 flatbar sitting around, so I cut it up and used it to mount the castors. Tapped it M8x1.25 so I could use old Datsun bolts to hold them on.
A note about taps… If you’re like me, you’re in the habit of reversing the tap a half turn every now and then to break the chip so it doesn’t clog up in the flutes and cause problems… but look at the photo of the tap below. Modern taps are designed to be used in CNC machinery these days, so they all have an angle on the leading edge of each flute… see it? This angle pushes the chips downwards at an angle so you can tap without any reversal of the tap direction. Doing a reverse turn with this kind of tap only causes undue wear.