If disregarding part 15 limits....

Curious, how much horizontal space do you have? I ask because you might find a T-antenna of interest also. Up 60' then out 60' on each side for example. Basically a top-loaded vertical with the "top-hat" increasing efficiency.

Alternatively, you might even try a vertical helical with top loading, too. I am not sure it would be any more efficient, but I suppose it might be somewhat easier to match depending on how low in frequency you intend to go. YMMV.

Either way you should be considering the other "half" of the antenna, too. As in the radial field if ground mounted, or you might even try helically wound radials if in the air.