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Are the stock plastic clipinabox thingys a bit rubbish, or am i doing it wrong?
Not actually clipping very well, getting stuck, taking up space inside where I want to put my luggage.
I'm minded to shorten some U-clamps and use a dome-nut, penny washer and gasket arrangement as an upgrade.
any experience?
Assuming you're talking a about a Ranger 90 softbox then .....
Are the stock plastic clipinabox thingys a bit rubbish, or am i doing it wrong?
Probably doing it wrong.
Not actually clipping very well,
I've never had a problem getting them to grip securely. There is a knack to it though. Make sure the red locking tab is up before you start. Push the U bolt up from underneath while pushing down on the clamp from above while twisting it. You'll feel it grip the thread as you twist and pull the bolt up. Make sure that the base of the clamp is seated in the holes properly. Lock the red tab in place when it's secure.
getting stuck,
This has never happened for me.
taking up space inside where I want to put my luggage.
This is a bit of a problem . I tend only to put soft stuff in it though which squashes round the clamps.  I've found that 4 big drybags from Aldi fill it perfectly.
All in all I've found it to be a great bit of kit and have used mine flawlessly for the last 10 years or so.