Fireplaces, Heating Stoves, Flues and Chimneys - chimney cap: will just any old one do?

I've always thought it a good idea to get one but since i just got a new boiler, maybe now is the time. Looking up from the yard, it appears to be somewhat rectangular shaped. I dont do roofs so would have someone else go put it on

so my question is, do they come in relatively standard sizes and can i just run to home depot and pick up any old thing they have? I assume somehow they have to adjust a bit and somehow clamp on the chimney to prevent winds from knocking them off.

Yeah, the generic ones from the Big Box have some adjustment. You center it and then equally tighten screws or bolts to clamp it. You still need to know the approximate size...

Chimney Caps and Brushes (http://www.lowes.com/lowes/lkn?action=productList&Ne=4294967294&category=Chimney+Caps+and+Brushes&N=4294959248&cm_mmc=search_google-_-Building%20Materials-_-Chimney%20Caps%20and%20Brushes-_-chimney%20cap&semsite=google&sembrand=n&semcatg=Building+Materials&semsubcatg=Chimney+Caps+and+Brushes&semkeyword=chimney+cap)

Ok thanks, didnt know to splurge for a fancy one like Chimney Caps - shop online at Chimney Cap Design. (http://www.chimneycapdesign.com/)

My installer told me i had a lot of draft and suggested putting a cap on.