Sorry but all I can picture with a bungee rope at a Airsoft game is one end tide to a tree the other to your m8s rig (when he's not looking) and RIGHT GO GO GO GO GO
There really is no point doing a expensive loadout now becasue in a year or so you would of grown out of it! Seen it happen to many times! Take it from me, I am 14 and 5.1!
I think thats tuff like guns and BFG can be used on any loadout (within reason) and its just the clothes that change. If you have a fairly generic gun like a M4 or MP5 just mod it slightly (I'm planning on buying a new hanfguard and stock for my future M4 so I can spray one set to make it BHDy and keep one black for other loadouts)
TBH even without the growing bit I would still change my loadout often. But I would advise buying a new set of expenzive 5.11 or the like gear everythime you grow a bit, its just not cost effective