You might look into the "DDRR" type of loops , I think there are a couple of outfits making them .

Also, some 30 years ago I built a "Folded Dipole" of sorts on a winabago for a friend following the design of the old "squalo" antenna used on 6 meters but up scaled the thing for 40 with a motorized,gear reduced tuning capacitor across the "end's" , worked quite well on 40 and even down to 6 , sorry, don't have my notes from back then but he used the thing till he died about 15 years later.

May give you something to think about.

73, John