I built my own drum with much guiding help from people in this group and another site. It's not as large as Rasqual's, nor nearly as much of a technological marvel but it does work pretty well. I was lucky I guess to get enough right the first time that it mostly worked! From people here I learned enough to make the mods that really made it work well.
The drum is almost exactly 12 inches long and 8.5 inches wide (pure luck). It turns on a 43 rpm motor that I got off of ebay for $20.00 or so (learned here but it should be faster). Early pics are here
http://homepage.mac.com/sevenover/PhotoAlbum48.html the one mod I've done since then is to run the spit through the drum at an angle so that the drum 'wobbles' similar to the way the Gene Cafe does (self epiphany). I run it inside a Weber Silver Genesis B grill... three burners rated at 36,000 BTU / hr. (luck)
The sweet spot for this roaster setup seems to be 2 lbs 8 oz but it will do 3 lbs 12 oz if I push the heat a little bit. I almost completely ignore the temp gauge on the grill as it has no measurable relationship to what's going on where the beans are! It's a guide and nothing more. (for the three min. I am applying the most heat, that gauge can be pegged as high as it can go) I rely more now on the settings I'm using on the three dials and what time the various smell and sound events occur during any given batch. I got here by logging, logging and more logging of what exactly was happening when with what settings and batch sizes etc.
I consistently get to first no sooner than 9:30 (based on comments made here) and usually at about 11:00 or 12:00. I can easily stretch the time between 1st and 2nd (when I go that far) to be upwards of 4 minutes if I want to and end the roast between 15 and 18 min depending on batch size and bean type.
A few things need to come together to get consistent roasts; Rotation speed, drum size, batch size, roast environment temp, ambient environment stability, airflow around the beans, drum / roaster preheating. I've also discovered that all heat is not the same (Conduction, Convection, Radiant) and adjust my burners during the roast to aid in the thermal transfer process a the right moments. (I learned a bit at last years SCAA show from the Dynamics of Thermal Transfer Inside a Roaster class.. it was quite informative!)
I started with the ultra-slow rotisserie motor that came with the spit but that was far too slow (as I was informed here too!) to work well. Not enough bean agitation, no airflow. I used to open the lid to 'check on things but now I now keep the grill lid closed 100% of the time. I only roast inside my shed where wind and rain cannot effect unwanted temp variations on my grill and I pre-heat the drum for at least 5 min (on the first batch) before I put any beans inside.
To roast 5 lbs, you're going to need a wide drum, and a motor that is strong and fast enough to turn the mass properly. Some sort of fins or baffles inside the drum to make sure the beans ride up the side and tumble over each other is a must.
There is probably more that I'm forgetting in my older age but there is much more experience on this forum that will hopefully bail me out.
