I just bought a drum exactly like yours and plan to use it as a separator. Just a couple of questions about yours.
1, How did you measure acuurately enough the inside diameter of the barrel (since it tapers some) so you could build your baffle?
Measuring tape. It gets close enough... Seriously, I used a fabric tape, the one my wife uses for sewing, pull it taut. I am within 1/8" side to side, which is close enough for rock & roll...
2. Where were you able to find 5 inch fittings and flex hose? Are the fittings PVC or metal? Lowes has some 5" metal stuff, I just bought a 5" to 4" mertal transition from them.
The fittings are 5" snap lock steel from Lowes, the flex hose is from Grizzly. I do NOT like the particular flex hose, it was cheap enough, and it does the job, but it is very stiff... I would rather get the clear stuff instead... Save yourself the money and aggravation of using the black hose and use the more expensive, but MUCH nicer to deal with heavy clear vinyl hose... Oh, get a HVAC duct crimper, you will need to crimp down the fittings a bit to get 5" to slip over it...
3. How did you seal the fittings? Silicone?
GE Clear Silicone Adhesive Caulk. Make CERTAIN you clean the Snap Loc pipe with vinegar, or rubbing alcohol prior to using the sealant on it. The pipe is coated in some sort of oil that will prevent adhesion of the sealant if you don't clean it off all the way...
4. On the side entrance, does the inlet pipe extend into the barrel any or just end at the cutout?
It is flush with the side of the barrel.