Although the original version by HPaul is great, I've tried to simplify it further. I've made just two stands - one left and one right - with deep channels. Also as I use foamboard for other projects, I have incorporated features that allow the stands to slide onto 5mm thick foamboard. Of course it can be used on anything that's 5mm thick. I use foam sheet as the background which is soft enough to slide in without any tension.
UPDATE1: I have created a further centre support - with integral foamboard clip - that can be positioned anywhere between the two end stops, to give added support when a large background sheet is used. This centre support uses the same radius as the back radius on the end stops, so they are all in alignment with each other.
UPDATE2: I have created a smaller set to hold A4 sheet (or thin card) for seamless background. There are only two files for this version - A4 left and A4 right. No support is required or any raft. The photo of these shows them one up and one down - don't be confused by this. Print both files with the closed ends seated on the print bed.