The RSP-1a uses a single SMA connector, and it isn’t really suitable for just poking a wire into. It’ll keep falling out and knowing how eager and clumsy young boys are there may even be damage.
Also, multi-strand wire is asking for trouble when ‘poking’.
You’d be much better with an SMA (male) to open-end adaptor. This is an SMA plug, ready fitted with a length of thin coax which has wire ends to which you can connect your 18-gauge wire and an earth connection.
Or you can get single-piece adaptors that are SMA (male) at one end and BNC (female) at the other. These are much easier to poke wires into if you must.
I suppose a compromise would be to get some solid, single-core wire and connect a short length of it to the 18-gauge wire, just a few inches so you can poke it into the RSP1-a without stray strands going everywhere.