Can't print an open frame properly, Maker and Replicator fill it

I'm going bonkers over the last two days trying to get Makerware or Replicator G to print an open frame model properly. Here is a link to what I'm working with:

http://www.thingiverse.com/thing:369109/#files

When I load the model in Makerware, it shows that the center frame is open (which is what I want). You'll note that even Thingiverse shows the frame open. But when I preview it or print it, the software wants to fill the center frame in. I've done everything I can think of in Sketchup to make sure that center frame is open/deleted/erased, and it appears it is. I'm sure I must be missing something simple, and I'm hoping someone here might be able to help guide me in the right direction. Thanks!

Netfabb says the .STL file has errors. You should try to figure out what in your design is causing the problem, but NetFabb can fix the errors. And .STL errors do cause all kinds of issues with the wrong thing happening on your prints.

Go hunt down NetFabb Basic and give it a try.... I suspect your print is going to turn out OK once you get the errors in the .STL file fixed.

Yeah, I have netfab already and noticed the errors. When I tried to fix them, netfab filled in the window as well. Maybe I'll try to fix error and then use netfab to cut the opening back out.

I pulled the part into NetFabb (basic) and attempted to repair it. I understand the problem you are seeing.

Usually, when I have run into this I am putting two things at a boundary and not intersecting them. Like if I put a 1 x 1 x 1 cube at the origin and the try to put another 1 x 1 x 1 cube at (1,0,0) to make something that is 2 x 1 x 1.

Can you check if you have two 'faces' at the same coordinate?

I appreciate the advice, Roxy. I seem to have fixed it thru trial and error. Initially I was creating the model by "sandwiching" two rectangles together. This time I went back in and created it from scratch but this time simply using the "pull" tool to create the window in the middle. Now it previews just as intended in Makerware and I'm going to give a go at printing it right now.