Have been using Jon Lawrence 'Printrbot Simple, 300mm Y Axis-No Sag!', thingi
http://www.thingiverse.com/thing:242185. With the 'ExtendYFront2013Fan.STL'.
The top rail feels loose now so I decided to tie it on with a block and screws.
Whilst about it I put a bolt holder and made a switch holder so that I could reverse the Y-switch from front to back.
Printrbot seem to have made this change, and it's probably easier to go with, rather than resist. I thought it more logical before, cartesian co-ordinates, bottom left etc.
You don't have to take anything apart to fit this.
Anyway, the frame feels more solid on the top now.

I tapped the two holes in the main block, 3mm, what else! These holes go all the way through, and if you have longer screws you could go all the way through it, and have a nut at the end. I used 20mm long screws. They felt long enough. UPDATE: 24/7/14, having used this or sometime now, I would recommend getting long screws, so that the whole thing binds together, but that might be just my problem with too low a temperature during printing. I used 50mm long screws, fairly sure you could get away with 40mm.
The clamp is a snug fit, but there are screw holes on top if you need them. I didn't in the end, it felt tight enough.
Put a lock nut on the bolt to counter vibration movement.
I used the old switch from the Z-stop, not used, now I have auto-level probe, the current Y-switch can stay where it is for now. It's very tight in there! I used the original screws too. Forcing them to cut a thread, half a turn forward, quarter turn back.
I was able to slip a cable tie at the front end to hold it down, but thought not everyone will be able to, so have uploaded a long front one that can be tied lower down. I haven't tried this, let me know if
it's a problem.
The cable can go down beside the Y-motor, and take that route, to keep it tidy.