For us, we wanted as low a profile as possible with the bed as it's going to mostly be used as a spot to lounge on and it's nice to be lower slung. But trundle beds tend to have a really tall profile due to the need for an extra tall span running along the opening for the lower mattress and all it's required debris. We're opting for futons with ~ 6" of loft. Casters are around 1,1/4" tall.
I talked to dad about it. he did a few quick sketches and sent them my way, recommending an aluminum I-Beam construction for the span. His calcs indicating that 2, 2" wide by 1/8" thick soft aluminum straps in a 4" I-beam should support 300 pounds of human pretty well along a 6' span. So it goes.