Yup---That's an oldie!!
No top vent back in 2005.
That was 4 years before I bought my first MES.
I know what you mean about wanting to go without any kind of mailbox. All 3 of mine have been working fine without one, and the AMNPS works great right inside any of MES units.
All you need is a decent adjustable top vent like you mentioned.
Make sure you put it close to the back "Left" corner of the top. Don't put it in the top right, like Masterbuilt used to do with all the Generation #1 units.
Bear