Hey Steven, I have not tried them myself yet, but check out https://kingkho.com/en/ .. I think some German guys do that.. the page looks quite well done.. found them on google maps when I was searching for mini storage in Hanoi, click here https://goo.gl/maps/R5Qr9smY1XE2 or search for "King Kho mini storage" .. maybe share here once you tried them..
@RudiHess_Luzern thanks for the tip. I rented an M unit with them since April or so this year. Their facility is good.. they have guards, you need a pin-code to access the building, lots of cameras, and so on. Similar to what I know from the Netherlands.. thanks again!