Long time back I needed much the same as you, only I had to haul it on the road as well. Made the frame out of electrical "unistrut". Four feet wide, six feet long, lift-off sides which doubled as ramps. The tonque was "A" shaped to deflect trees in the bush and hinged so the frame could tilt.
I've learned that wide tires are good, but tires with a large diameter are better at rolling over rocks and logs. By using the rear suspension from a VW Rabbit I got good ground clearance (no axle) and mounting the tires just outside of the frame gave me suspension movement. With the trailer hooked to the ATV one person can lift the rear and move it to the side, yet it still carries a 700lb bike down the highway or a 1000lb moose thru the bush. (I have a winch on the rear of my bike, I just winched it onto the tilted trailer.)
I've owned and used this trailer for over 30yrs now and other than replacing the plywood floor and tail-lights its done a great job.