I’d try making a suitable image of 16x9 format (1920 x 1080 px or 1280 x 720 px) with a narrow black border all round and install that as a background texture. Then make each of your scapple “slides” within that frame and take a screenshot (Shift-4 to define the area) and resize in an image editor as necessary.
Here’s a corkboard 1280 x 720 px version. Took a couple of minutes using Graphic Converter!
A Scapple canvas has no limits so you would still have to make copies of what is seen on the screen. Either having one Scapple document per slide or use Xiamenese’s suggestion and have a framed texture which tiles itself over the whole canvas and zoom in on each in turn.