Is this the one where a velcro strap goes thru a stainless rectangle thing so it doubles back on itself? And you don't have to open the cover to adjust - just rip up the velcro and pull it tighter? Bit like velcro straps on kid's shoes?
If so, they are not much good as u can't get much range of adjustment.
Better is the old school one - where the cover is split lengthwise at the top, you open that up and there is 3 layers of velcro inside which makes it adjustable plus or minus about 3 inches. The first system I mentioned is lucky to adjust about an inch.
I have 2 or 3 y/o Stuart Bell ones like latter, and the Starboard ones like the latter (on 3-5 y/o Evo's etc) and they are by far the best IMO. Sometimes the original design is the best