You need the cut-outs because throttles break quite easily and will stay on when broken - not off.
Not a problem though. It's very cheap and easy to make brake switches.
You buy two of the cheapest cycle computers from Ebay:
LCD Digital Bike Cycling Cycle Bicycle Computer Odometer Speedometer Velometer | eBay
Cut the sensor wire close to the computer and chuck the computer..
Cut the connector/s off the ebrakes about 2" from the connector.
The sensor has two wires. Some are coaxial. Separate the wires and join them to the two on the connector - preferably by soldering.
Cable-tie one of these sensors with connector to your cross-bar or anywhere where there is bare brake cable running along your frame.
Fix the magnet to the cable nearby so that it slides towards/over the sensor when you apply the brake. Fix the magnet just like fixing it to a spoke, but not so tight to damage the cable .
You have to experiment with positions until you get the switching at the right point.
Front brake can be harder to find a suitable bit of cable. You can use the bit between the calipers, but then you need to make a small bracket to hold the sensor.
Just having a switch on the back brake should be adequate.