Get your bathroom scales and place the board behind them - this should have the corners rounded off.

Put the whole lot inside your plastic bag. Wrap any excess bag around the scales and secure them with some sticky tape.

Put the pipe from the vacuum cleaner into the mouth of the bag and use your hands to wrap all the excess plastic around the tube.

Turn on the vacuum cleaner and see what value the scales read.† Some types of electronic bathroom scales are slightly too clever for these purposes. They will only actually give a reading once the force on them has stabilised. However many of these scales have a mode for calibration in the factory. Inside the box which holds the display there is often a black connector with two pins sticking out of it. This is sometimes behind a hole in the casing so you don't even have to remove any screws. If you connect the two pins together and release them, the scales change mode, and normally the first mode you come to is more conventional.

† Some types of electronic bathroom scales are slightly too clever for these purposes. They will only actually give a reading once the force on them has stabilised. However many of these scales have a mode for calibration in the factory. Inside the box which holds the display there is often a black connector with two pins sticking out of it. This is sometimes behind a hole in the casing so you don't even have to remove any screws. If you connect the two pins together and release them, the scales change mode, and normally the first mode you come to is more conventional calibration mode - beware this often disables the auto power off feature. To get the scales back to normal just pull the batteries out an replace them.