With the info from Mike and Dave I started by wrapping the frame sections in paper and tracing all holes, so I always have a pattern as "backup" when it turns out I have welded up too much holes. Than it was basically a matter of welding the holes up and spending some time grinding everything smooth. The last bit took a wee bit longer than expected, because the weather suddenly improved and I didn't want to bother the BBQ-ing neighbours with my grinding.
....So i had to wait a few days for the temp to drop and everyone to go inside.
Alex