Tuomos,

I'm guessing my model weighs about 15-16 lbs without the fan and batteries. The dynemax fan and 12S packs add about 6 lbs. Pondus offers a very valid option, and that's to run a Stumax SM100-45 fan and you may not have to modify your current ducting. What is the dia of your ducting? Do you know the estimated inlet area?

If you ran the SM100-45 with Neu 1521/1.5Y motor (that's what I have in my Skymaster Sabre), you could run it on 11S and that would save you about 0.9-1.0 lbs vs. the dynemax on 12S.
On my sabre this fan pulls 5870W at 148A peak and runs at less than 3000W (67A) for most of the flight. The ducting is greater than the fan requires and is straight in and out vs. the 104 but it still makes great power. The other nice thing about using a smaller fan is that you may be able to fit two 3S packs back near the fan, and only a single 5S pack infront of the inlets. I can take measurements if you'd like. The other nice thing for balance is that there is space with that generous fan hatch to put the fan backwards. In fact, I've just made up an adapter for mine to move the fan back 8" so hopefully I can add larger capacity packs.

Here's a couple pics of the SM100-45 in the Sabre. That plane weighs just under 18lbs rtf. You can see the holes where I had originally mounted the DS94 fan (this turned out to be too heavy - 25lbs on 14S - and the inlet area wasn't ideal for it) and going with the 100mm fan gives lots of room for the batteries on each side of the inlet.

Hope some of these thoughts are helpful. I got my Robart retracts from Aeroloft as Robart has discontinued them. I have been able to purchase most of the parts for the gear from robart so they may be able to put a set together if you asked.

Btw, if you pull the used scale gear out and wish to sell it please PM me. I don't think this will be my last 104.