With that long of a trailer those scissor jacks at each corner will do very little. You need JT type diagonal bracing at both ends. Plus some form of a jack under the frame near the axles.
You can go cheap with just a piece of angle iron and clamp it to the frame and down to the opposing scissor jack. You will need on the front and the back.
Then get some cheap scissor jacks and attach then to the frame in front of the front wheels. Those will take out the frame flex. Or you can buy some cheap screw jacks from Walmart and set them in front of the wheels.
To do the ratchet strap thing just place one hook on the frame rail and then hook the other side some where on the base of the scissor jack. Crank it pretty tight. You want the front and rear diagonal bracing to go in opposite directions so they work against each other.