Sorry, no offence but people with way more experience than you, in both biking & airsoft are giving you sound advice, but I'm getting the feeling that you believe you know better, as the younger generation often do.
for starters, have you ever ridden with a sidecar ?, as a learner I'm guessing not, even a proffesionally sorted set up will mean your whole riding style will have to change, imagine a quad with a bent frame & more weight on one side so it will try to "crab" depending on acceleration/speed/road surface & turning.
then factor in your low weight, your bikes low weight & the extra weight of your kit in the "car".
finally, your on a 125, 15bhp at the most, 12 is more likely, the extra load & stresses will tear the guts out of it in no time, you'll end up with a pile of scrap.
& don't get me started on cheap gun bags or scabbards, your kit gets exposed due to cheap solutions, your ass in nick etc because of it.
in your shoes I'd do one or more of the following:
take the full bike test, in order to get a machine more suited, something "dual purpose", look it up.
or sell your current steed, get a DT or similar that has a higher frame profile enabling more luggage options.
get a decent hard rifle case, nuprol or star etc, & get someone who knows what their doing to make some decent mounting mods, so if the plod ever have reason to get involved, you can show you've made reasonable attempts to hide a secure your gats.