those are good:
http://www.ebay.com.au/itm/Holograph...l%3FMyEbayBeta
I have two of similar (one is larger, and both have round lenses) but whichever you buy, you will not miss.
Those plastic are also OK, because they are ligh-weight.
http://www.ebay.com.au/itm/200620851...84.m1423.l2649
http://www.ebay.com.au/itm/Telescope...item2c61d5e7b7
Stay away from those:
http://www.ebay.com.au/itm/Tasco-Red...item256b83f8bd
I have one and it is not accurate, the red dot moves too much away from target in one corner (away from LED.. because it is too short for the size of the reflective lens)... I am not using it any more.
EDIT:
I replaced the resistors in those metal ones, their values were too small for night use (LEDs were too bright. starting from 22k and lower I obtained suitable illumination.