Nope, doing that now.

=edit= now after closing down to 200 F.

http://www.bijlsma.cz/stuff/uds005.jpg

Is air able to get under the bottom side of your basket? It's not solid on the bottom is it?

The bottom is solid, here's a better pic:

http://www.bijlsma.cz/stuff/Basket.jpg

Should I drill holes at the side of the bottom pan ?

The only thing looking at your ash catcher it almost seems like the sides are at the bottom level of your charcoal grate. Hard to tell by the photo, are your bolts from the bottom of the catcher 3" above meaning 3" from the bottom of the catcher to the grate? If the sides of the catcher are really close to the bottom of the grate looks to me you really don't have much airflow once the coals ash starts dropping. If you want to keep the catcher I am guessing drilling holes around those sides probably ain't gonna hurt and actually I think your problem is the more I look at it - airflow. I used a 16" pizza pan from Wally World which has very little lip on it and I think it cost 3 bucks or 3.50. Keeps the basket lower as I bolted thru the bottom of the pan and with a bunch of washers and nuts the grate is 3" from the bottom of the pizza pan so the pizza pan just sits on the bottom of the barrel with the bolt heads holding it off the bottom.

Correct, the edge is same height as the bottom of the basket.
I'm gonna drill 3/4 inch holes around it now, buy some new charcoal and report back in about 1 hour from now.

Thank you, much appreciated.

Drilled all the holes around:
http://www.bijlsma.cz/stuff/uds006.jpg

Made a small hole in the middle and put the red hot coals in.
http://www.bijlsma.cz/stuff/uds007.jpg

Within 15 minutes, it went up to 240 F.
Added also two small pieces of applewood:
http://www.bijlsma.cz/stuff/uds008.jpg

Half hour later at 280 F :-)

= edit =
Another half hour later at 320 F :-)
Me so happy, looks like it works properly now, but still, any comments / advice for improving are very welcome !

== edit ==
Another half hour later still at 320 F.
I guess this is the max temp then
To try to get it hotter, i closed 1 of the 3 pipes. The daisy lid was all the time closed.

Nice UDS Lets see how it cooks

Your basket that's not galvanized steel IS IT? Also have you checked your thermometer in some boiling water?

Euhm... Actually, i don't know.. the bottom of one of those cheap grills.. When i pushed my drill in it, the paint or whatever it is, jumps off.
The sides is made from 2 layers of fence material. I used a flamethrower on it, it will get red hot, but not melt away. Further it's quite flexible.
Is that a bad thing ?

Uh oh, the galvanized police are patrolling!

I thought all outdoor fencing was galvanized. (Guess there is no such thing as indoor fencing?:-D)

Here is what I did
 

Not policing just asking! It's his UDS I am sure he has all the info on galvanize parts

Galvanized parts are bad for using....?

If its galvanized and I am not the galv police I would not use it and most likely it is if its outdoor fencing as the outdoor fencing usually is treated so it doesn't rust. Now does that mean anything will happen to you based on how high the temp gets in a smoker, probably not. If you were welding it thats a different story and if you used galvanized bolts dropping them in vinegar works, takes a few days but it works. I used zinc bolts for my basket stainless for the cooking grates but I did drop the zinc bolts in the vinegar for a few days, I was just being cautious. I used expanded metal which most on here do, if you have a home depot they have a 12x12 piece here anyway for 10 bucks. I took my skill saw and cut it in half so I had a basket 6" high had to bolt the 2 pieces together as my charcoal grate is 13.5 diameter I think, you have enough to have about a 1" gap from the side of the edge. I just used ties from copper wiring to attach it to the grate, works fine and half full of charcoal last weekend doing ribs it burned better than 5 hours and more if I had not shut it down. Not an expert by any means, I figured drilling the wholes would solve your problem one picture looks like you didn't drill all around the edge if you didn't I would do that. If you get to 320 not much would be cooked above that if anything, its a smoker not a grill.