I had an early steel .25FP-S, completely box-stock, that used to pull a 36 oz. flapped profile own-design (span 49", area approx. 480 sq. in.) very nicely, turning a grey Graupner 9x5 on 5% nitro. From what I hear, the .20 is just about as powerful, so any smallish .35 design, built light, should fly well. If you'd like a full-fuselage classic, how about the English 'Scimitar' design by Bill Morley? Nice-looking model, originally intended for an unsilenced Veco .19, I think.