In the meantime I took down the unfaired floats I had set away, and brought the surface close to the final finish before painting.
Here you can see the solution I came up with for the long board. A simple piece of plywood. Wrap the sanding paper around and that's it!. This way sanding paper can be used up nearly 100%. I had made some custom long boards with handles and everything but it turned out the paper was ripping off prematurely leading to significant waste.
As I shopped around looking for a orbital sander I wasn't to eager to pay well over €100 for a decent 150mm disc sander. Luckely I found this 125mm bargain for less than €50 with variable speed and just about as effective as higher branded products. Since 125mm discs are more expensive and harder to find in different grades I ended up using 150 mm discs which work perfectly.