The video shows linear rails, which are a great help if you can afford the long lengths. The video example does not bind with the belts low down on the sides, and looks like it would cut OK too.
Linear rails are a completely different animal and can resist huge loads, so any additional loading due to poor design can be accommodated. with the simple bearings this is not the case.

What do you mean by rhs instead of channel? If this is RSJ then this is also poor in torsion and would not make the best gantry. Run with the box section.
I've been an Engineer for about 16 years, and into DIY cnc for about 2 years, so you can decide how to take any of my thoughts on this one.
bit worried now:surprised:... rhs is rectangular hollow section and although an rsj (not been called that for a while, UB or UC now) is poor in torsion, even the smallest size would be perfectly adequate for diy cnc applications..........

how to make Friends and influence people eh:heehee: