Hey Steve,
I don't know which way you are adapting or why but I had a problem with cooling with my firewall mounted cooler. I had originally used the square cut Van's 3" fitting at the oil cooler. I decided to try a better transition so I made my own out of fiberglass. You may be best making your own as well. Simple to do.
I made mine out of 6061. I bought a 4" diam AL tube and 040 sheet from Spruce. I cut a slice of the tube, cut pieces of the sheet to make a trapezoidal box. I had all the pieces welded together at the fab shop at the Air Force base I was stationed. I was going to get 063 to make it easier to weld, but the guy said he could do it. And he did a fabulous job. 7 years and 850 hours later, it's still like brand new!
Jerry Esquenazi
RV-8 N84JE
Here are a couple of pics. The flange on the baffle was done in the same manner.
I have seen a similar thing done with aluminum by riveting the pieces together, either by bending tabs and/or using some .032 angle.
Using ABS filament you can throw the part in a jar along with a small cup inside it filled with acetone, then you put it back on the printer and use the heated build platform as a hotplate of sorts. The acetone vapor will smooth the part.
I have a Reprap iteration 2 and printing with .2mm layer height with a .35mm nozzle I get good surface finish on outside perimeters.