I made my own since the commercially available ones aren't exactly inexpensive.
A big consideration, one which I did not think of until AFTER I made them, is what Rob stated. As shown in the pictures I rounded off one end and did so equally around - a bad, bad decision since it created the 'hump/bump' that Rob described.
I had to make new ones - luckily I tested after I made just 3 so I was quick to discover the error of my ways!
As you can see in the last picture, there is quite a difference in heights - I landed up putting bracing under the O27 track.
Another consideration is that unless you keep all of the O27 track at the same 'O' height, the O27 track will dip down to table level creating a bit of a 'ramp'.
- walt