Have you tried reducing the % of support? Cura default is like 20%. Try reducing it to 10 and see if it makes it easier to get it out. You could also try printing it laying flat on its back instead of upright and see if that makes the supports easier to remove.
For removing supports, I usually use an Xacto chisel or some other kind of thin tool that I can get in there and wiggle around to break the supports free. If they're in there too strongly, then reducing the support percentage would be the first thing I'd try.
I dont think you'll be able to print it support free, the roof of the cavity wont have anything to stand on and will just fall in. The printer cannot deposit filament into thin air. The only way I can see to print it support free is to either separate the roof of the signal and make it a glue on part, or lay it on its back, with the signal box face separate, so you can print it as an open rectangle and then glue on the face. If you take off the face and print it laid down, then you'll still need supports for the step from the base to the signal, but that ring of material should break off fairly easily.