That lower "DIN" slot is a tough place for an indash LCD. Believe me, I tried it myself a long time ago.
Your best bet is to ditch the stock headunit and get a standard 1 din receiver with an AUX input and put it in the bottom DIN with the LCD in the original location.
Its funny, your setup is almost identical to my first one from about 3yrs ago when I had a Compaq laptop running under the drivers seat with the exact same docking station!
I think the pics in my sig still work you can see what it looks like how I described it.
My new setup is completely different though, I hacked up my lower Din/Gearshift Trim/Cupholder and slammed an 8" touchscreen in there, pics to come soon.
Click Here to see my Car's Website
2003 Nissan Sentra Spec-V