Do you have any trees nearby? If not, proceed with the helical plan. I would attempt to make the capacitance hat at least 6 feet square if at all possible.

Should you have trees...build a center-loaded, cage T or Y-top. The vertical element length will be determined by the highest set of branches into which you can get support ropes - and the horizontal length (capacitance hat) should be at least 20ft per side. Cage construction will be used for the vertical portion of the radiator in order to maximize bandwidth; 6-8" spread by means of Plexiglass or similar homemade insulators spaces 3 #12-#14 AWG stranded wires. Use a thicker one for the very top, where the capacitance hat begins as it'll be under a good amount of load.

Figure out the horizontal/vertical dimensions along with radiator diameter then plug those into your favorite antenna modeling program and find out what you'll need in the way of a loading coil. I would use a piece of Pi-Dux in as large a diameter as I could find; 1" to 1 1/2" OD PVC pipe will serve as a coil support to attach the vertical radiator sections. Model the coil placement and resonant frequency so it's just a bit higher than "center" and is at the top of the band segment you wish to use.

Next, use the same model to figure out how much of a base loading coil you'll need to cover the lowest operating frequency on which you plan to operate. Build the load coil using similar PiDux/PVC construction then source a few surplus vacuum relays. These go into a waterproof tuning box, are connected to tap points on the coil and are used to progressively short it out via shack-sited controller as you wish to go higher in frequency. Feed the antenna at the bottom of the base load coil.

I would also stagger-tune the radials instead of cutting all of them for one frequency. This goes for both self-supporting and tree-supported versions of this project. Key to making this project work is to put lots of them down, of course.