Here's a dead-simple one built from one of those fishing swivel seats, a pine board, U-bolts, a 1-ft. strap, and various screws, washers, etc. Two seasons with no problems.
The u-bolts should be sized for your frame, and for an aluminum frame, to center the seat I'd get a couple of collars (SpeedRail, etc) rather than setting sheet-metal screws in the frame tube as I did here. **
**Actually, I think I shot the pic before setting the screws, big fatheaded 1/4-inchers which are just inside the u-bolts.