US Stateside, we use a lot of "White corrugated Mailers" from big mail order Office Supply stores in packs of 50. They come in (imperial measure here) 2"x2" (51mmx51mm)x (4", 5", 6" thru 12") (also in 3"x3"). They are (very) Heavy duty, and I modify the bottom flaps by taping then up to the sides (trimmed)so I have a flat area for the doubleside carpet tape. I'm sure there are some Big Office Supply stores over there (here there is: Office Depot, Office Max, Staples, U-Line etc.)