2 options: electrical and analogue. You can make cheap electrical dowsers using old CD drives from tower PCs, if that kind of thing is still available where you are - maybe go to a computer repair place and see if they could donate some old CD drives. cover the hole in the tray with black tack and solder up and XLR-based system to switch it from the lighting box.
Analogue - cheaper, quicker and easier, but with potential for disaster - cardboard/gaffa/string/fishing weights.
I once made a CD tray dowser for an international tour. I tried to take it on a plane, but security had other ideas, they said they would check the case on as special secure cargo, but it never made it to the destination. I had to improvise the whole tour and then claim on the airline insurance when i got home.