Save the hassle, just don't.   Tactically sweep the house instead, few laps up and down the stairs to see how it fits and she'll be right.

Definitely don't load it up with game day kit, while its technically not illegal, if a concerned member of the public reports concerns & you're stopped & searched, weird questions will be asked & you, & by definition, airsoft, will be looked at unfavourably.  Plus while the kit isn't illegal, pretty sure there are laws regarding causing fear etc to the public, which could be used to charge you. Don't leave the house wearing it, travel to a suitable area where walking/running is the norm, & load the vest up with tins of beans or bags of sugar, more than enough to test its suitability.   Or you do what we did in the forces, adjust to fit, load up with kit, & jump up & down in your "billet", if it feels secure, leave it til gameday.

Cool bag with lots of water / coke / monster etc.     Tactical emergency Haribo.   Comfy trainers (mostly as I can't drive in my boots)    A couple of t shirts or long sleeve wicking fabric type tops as you will really feel the cold when you stop running about/sweating.   Spare eye pro.    Spare battery.   Depending on site I sometimes take a camping chair if there's limited/no seating.   Barrel sock as some sites say its mandatory.