After reading this synthzone thread with great interest, I finally purchased a RockNRoller (model R8RT) cart, and LOVE IT!

Thanks to Gary Diamond's pictures, videos, and construction guidance,
I had a pair of aluminum rails constructed for my cart secured by quick release detent pins.

Here are pics (see below) of my RockNRoller Cart with custom aluminium rail assembly of which includes the option of mounting the rails at 3 different height positions.










The 2 upper positions utilize existing pre-drilled RockNRoller handle holes. I had a 3rd set of lower holes drilled so the keyboard case can sit below the top of the cart handles (see pics). I
purchased two 5 ft lengths ($11.77 each) of 6061-T6 1" Square (0.125" thick) Aluminum Tubing and four Stainless Steel detent pins with Lanyard...452; $4.59 each.
The Aluminum machined cuts and holes were all done by my local machine shop so I don't have those specs.

The big advantage of installing the light weight aluminium rails (shelf) is you avoid the risk of back injury not having to bend to lift the keyboard off the floor. Gary, both my back & myself thank you for the help and the pics & info you've shared.

Scott