Since the ADA was passed, there is a possibility that your landlord may be required to install the ramp for you. Some requirements are regional, some national, so check with your local authorities.
The ramps aren't terribly difficult to build...especially a small one like you require. Go for the minimum slope with a sufficient curb to prevent rolling off the side of the ramp. A siderail may or may not be required due to the small rise.
Get in touch with your local Boy Scout organization, hunt down the "Scoutmaster." We do 2 or 3 projects like this a year, whenever requested, FREE of charge to the homeowner. Typiclly a lumber yard will donate the materials and the scouts provide the labor...with proper supervision. They are built well, to a high standard...the kids are very consciencious on projects like this.
For lightweight/portability, you may want to consider a metal frame with an anti-skid/diamond expanded metal surface. We did one of those as well...again, free of charge.