Clone/Create new Shipping by Weight module problem

Hope this question isn't too, ahem, simplistic here...

I'm trying to create a new shipping module where shipping costs are determined by weight. (Basically, we want to have 2 'by weight' tables... one for airmail and one for surface mail).

After doing a search through the FAQs and the forum, the closest I found to telling me how to clone an existing module was here: http://www.zen-cart.com/forum/showthread.php?t=41751

So, I took an existing php shipping module (and the language files) and made copies. I also edited the names of the module. However, a couple of small problems have come up:

• When I go to admin->modules->shipping, the new module shows up as blank
• When I try to install the module I get the following error: Passed variable is not an array or object in C:\Inetpub\wwwroot\zen\admin\modules.php on line 279

Is there something I've missed? Or better yet, is there any other documentation about how to create/clone shipping modules that I've overlooked? Or does anyone have a better way to go about things?

Thanks in advance.