As always I start by checking my Panzer Tracts. There are several drawings of Sd.Kfz.251 Ausf.C provided by Hilary Doyle in both No.15-2 and No.15-3. In fact the ambulance version shown in your second photo, mittlerer Krankenpanzerwagen (Sd.Kfz.251/8) Ausf.C, with a welded armoured body, is the subject of one of these drawings. The first thing that makes me suspicious is that tabs welded to the armour, through which the canvas cover is attached, are more or less the same for all Ausf.C and quite similar for the Ausf.D. These tabs are still to be seen on the model of the ambulance that you posted - but they are not being used to hold the cover?
Searching through Sd.Kfz.251 photos it is hard to find a vehicle where the canvas is fully installed. None the less there are many pictures (even in PT No.15-2) that show them partially in place. In every case the canvas is held in place by straps sewn to the canvas cover which are then looped through the tabs. When in place the canvas comes down over the side of the armour. The front of the canvas seems to be held in place by the rain guard arc on the roof over the driver.
So in my opinion the canvas shown in the photos that you posted has never been researched and is just to make the model look pretty. You will need to create your own cover. It would be interesting to hear what others think.
Attached is a photo from eBay showing an Sd.Kfz.251/8 Ausf.B. While not great is illustrates the points I found in my short research using reliable sources.