OK, d00d...don't say I never did anything for you, bro... 
Here's what you'll need to mount either the 2820 or the Zumo: 30997MC Mount (the C indicates chrome, of course) will get the handlebar clamp, the stem, and a backplate. The accessory chrome backplate with the AMP hole pattern is part number 60991C. I have not had a Zumo in hand, but believe that it comes with a cradle.
Hi,
I own a Zumo and also very interested in buying the Techmount (since I found this thanks to you guys

) which I find more "Harley style" than the original RAM mount delivered with the Zumo.
To make sure that the mounts you are speaking about are those I need, I sent an email to Techmount and here is the answer :
Philippe, the 30997mc will bolt right up to the 4 hole bolt pattern on your Zumo cradle. You do not need any other parts, but you might want part # 60003, which is only $14.95 and makes it very easy to take both the cradle and the Zumo off your bike, if you want to run a radar detector or toll transponder, etc.
Dave Macaw
Customer Service Dept.
HighGear Specialties, Inc.
www.techmounts.com
sales@techmounts.com
toll-free: 1 866 341-4192
fax: 407 656-3799
local: 407 654-3322
I was wondering if you find the 60003 part a good buy since it is intended to release the cradle quickly but what about the wire? So I don't think this is an accessory I need for my case?
And what about part 60991C which, apparently, is not necessary according to techmounts specialist. I am not sure he (techmounts sales) is right since the top plate of 30997MC has only 3 holes that do not fit those of the Garmin cradle, instead, 60991C seems to have the 4 holes at the correct place. Am I right?
Last question, how is 60991C attached to 30997MC?
Thank you for helping me. I need to be right at the first order since I have to get the parts shipped oversea and don't want to send them back!
Have a nice weekend.
Phil