If you are using a 67mm lens adapter then you can't use your original Lens cap which has comes along with your camera SX camera (SX10, SX20, SX30, SX40 and SX50). So If you are going to by a 67mm lens adapter you need to buy a 67mm lens cap and 67mm lens hood (if you use a hood) along with it too. Good thing is these are not expensive. Also it's just a piece of plastic so you don't have to worry a lot about quality. 67mm lens cap cost only around $5 - $15 depending on you want a branded on or just a lens cap which works on your camera. Whatever lens cap you buy make sure it's a snap on type.
If you like to keep everything original and nice you can buy the Canon E67U 67mm Snap On Lens Cap. This will cost you extra than a usual lens cap so you will need to pay around $15.00 for this one. But it will look nice on your camera with 67mm filter mount. It works fine with both 67mm adapter and 67mm filters and grip well too.It doesn't come with a leash so make sure to get one and attach the lens cap to your camera. You don't want to miss place it which can happen quite easily.
There are unbranded universal snap-on 67mm lens caps you can find for cheaper at Amazon with free shipping. Cost will be around $5 - $10 so it will save you around $5. So if you are not interested in Canon brand or anything just go for one of those.
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It's same if you have a 58mm lens adapter and filters. You will need a 58mm lens cap and 58mm lens hood. These should be cheaper than 67mm items.
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You must have either 58mm or 67mm lens cap if you going to use filter adapter for your camera. Original one won't work at all. So better you buy one along with your filter adapter.