Hi Koi, take a look at these examples and note the spacing in the corner areas between the tabs. I think you're basically missing that spacing.

http://moi3d.com/forum/index.php?webtag=MOI&msg=8126.3
http://moi3d.com/forum/index.php?webtag=MOI&msg=10666.4

Look at the corner areas in particular, and compare to your structure which has material in the corners where I think it needs to be empty:



That's why it's not working quite right when you fillet things, if you had the empty margin spaces in the corners it would help the fillets to be separate and not to run into each other directly like you currently have.

Hope that helps! - Michael