I had the same problem and wanted to mount a GP 3" spinner with aluminum back. I could not find an adapter long enough for this.
I tried an adapter that mounted on the shaft, but this didn't work well for me. (something like this: http://www.allerc.com/product_info.php?cPath=36_61&products_id=2360&osCs id=0293fe585f8d29e9f8ddb8a1f00f1ab4)
. I couldn't get this to work w/o vibration, though I realize now that I may have had a bent shaft.
I ended up machining my own adapter with sufficient length to do the job. I made it to mount on the back of the engine. The first think I have ever made on my Unimat DB200.
This worked like a charm.
Bear in mind that the 3" spinner adds significant weight, and you will have to add ALOT of weight to the tail. (about 1/2 a stick of GWS clay! if using a 3S 2100 mah motor)
Also note that I think the added weight IMPROVES the flying characteristics of this plane. My landings are much smoother with the added 3" spinner and weight on the tail. I still have practically unlimited vertical with the EPYAYA high-performance configuration.