you can use a Karavan KB3100L..with add on axle
I have a Karavan mdl KB 3100-L trailer that I modified with an added tandem axle and I built a 8-foot long keel rest with a ten inch wide piece of U-channel HSS and then added four hull supports made out of out of 3/4 inch diameter threaded rod and square tube steel. (I have a stick welder and a cut-off saw). It has a gross weight capacity of 7000 lbs. The whole rig cost me $4000 new and took about 40 hours of my own work to modify. You do need a tandem trailer (for the stability in towing), and the whole rig will weight about 4400 lbs, as loaded. You will need a good-sized engine, at least 4.5L in size, too. The rig will tow well at 70 mph if you have about 350 lbs of weight on the trailer tongue, and if the tire pressures of the trailer are at about 45 psi. I do not recommend towing at over 20 mph on a single axle trailer as it will sway badly and overheat the tires. My trailer has hydraulic surge brakes, so no equalized hitch or sway bars are possible on that type of braking rig. There are pros and cons on electric vs hydraulic brakes, but hydraulic are less troublesome, provided that you get the trailer just right so that it does not sway under tow. If you have doubts about sway issues, ten use electric brakes and an equalizer hitch with sway bars. The hitch and sway bars will cost you about a thousand bucks extra, so I opted for the surge brakes and built carefully to avoid sway problems.