Is this a one off job or regular use? For regular use, the longer the ramp the easier the slope, but you end up with a long, perhaps too long, heavy ramp to contend with. There are loads of designs for van ramps for things like motorcycles, quad bikes etc., which you could look at for ideas.
For one off use, then yes, cheap and cheerful as you suggest should work. Maybe a couple of intermediate legs/blocks/wedges/bricks between ground and ramp. I have a sheet of old white conti-plas which isn't strong but is slippery, I use it in the boot of the hatchback if I need to slide heavy stuff to the other end. Getting a heavy thing up a plywood slope won't be easy unless its on wheels.
