Here is an interesting project that enables you to produce an efficient, yet short, vertical antenna that requires no radials. The concept and design is discussed by Peter G3OJV. Maybe you can adapt this idea to suit your own particular needs.

Flagpole DX Antennas
   • DX Flagpole Antenna for Real DX 1606...  

Ham Radio Store
https://www.hamradiostore.co.uk

LDG Antenna Tuner
https://www.hamradiostore.co.uk/produ...

Spider Poles
https://www.hamradiostore.co.uk/?s=Sp...