As requested, here are more details on creating the infinity curve to go with the previous lightbox tutorial. Cut your 22" x 28" poster board to just under 12" x 28" (or whatever width of box you are using). If the edges are catching on the sides of your lightbox, trim the paper a little more. It should easily slide in and out of your lightbox, so the edges don't catch and cause bumps that distort the even and smooth background.
Push one edge into the top corner.
Then keep pushing until the back and bottom are flat and you have the desired curve you want. If your poster board slips out, you may need to weigh it down, but usually I set up my camera on it and that holds it in place. By keeping the extra length in the poster board, you can adjust the radius of the curve by pushing it in or sliding it out. Take some test pictures and see what you like best.
To reduce the storage space needed for my new lightbox and lights, I cut the end off an 11" x 15" box and inserted it into my light box so it covers all three sides with the cut outs (with no infinity curve piece in it).
Then I can store my lights safely inside the lightbox without fear of the lights poking through the filters covering the cut outs or scuffing up my white poster board lined interior.
May the light be with you.
(Make sure you read this post with how to make the lightbox and lighting rig.)