i think there are a few options:
(A) maybe just maintain the current set up: two plastic sheets, BUT put a smartphone behind both plastic sheets… [no need for a thin red line on the smartphone]
(B) just one plastic sheet, PLUS one smartphone, separating by a few centimeters
(C) no plastic sheet.. JUST the smartphone. The service judge and the umpire have to check or calibrate the smartphone before the match with a ruler, the same ruler used to check height of the net before starting the match. This ruler, in addition to measuring the height of the net, needs to have addtional marker at 115cm so that the service judge (together with the umpire) can check and calibrate the set up. You do need the thin red line on the smartphone.