Ideas? Yes. Tested? No....
Won't be small, but you could use a brushed motor controller rated for more than the current you need (you don't want it to start limiting current at all cuz it'll drop the voltage to do so), with a large capacitor bank on it's output instead of a motor, to smooth the PWM output into a flat DC voltage. Might also need a series inductor between controller and capacitor bank.
Use a mounted on the controller for a throttle control, and set it so that the final PWM voltage is whatever you need your system voltage to be. Probably around 3/4 of full throttle, for converting 48 to 36. Then lock it off. Or use a pot to determine resistance needed, and then use a plain resistor. (or resistive voltage divider).
Some controllers have a "throttle high lockout" so if throttle is not at zero when they power on, they won't do anything. If you get a controller that does that, you'll have to have a switch between the system and this contraption, and use a throttle pot (or a switch between the voltage divider output and the throttle input), and set it to zero before turning it on, and then power up, then set it to the right output voltage (or flip the switch on), before switching on the connection between this and your system to turn on the bike.
Like the DC-DC's, this would also reduce your range by the efficiency of the conversion.
Personally, I think it would be easier to replace the actual controller on the bike to one that takes 48v.