If you bought that small trailer, added a healthy nose cone, covered and boat tailed the wheels and built a bit of a kammback/boat tail, it probably wouldn't be too bad. I wouldn't put a ton of time into it if you have the time and skills to build a properly streamlined trailer later on.
Otherwise you'll be looking at elaborate gap filling like aerohead and BamZipPow have done- it doesn't look easy!
