completely understand your predicament having a fair few MC's myself (and one girl thats over 7kg on her own!!)
i found the larger trees from zoo plus and from mczoo.co.uk are fantastic - preferably the ones that have larger bottom bits to keep the structure safer xx
i have many many of these in my house which are used on a regular basis and they all handle the weight of a few MC's xx
if you find any you like let me know, i have several different ones and if i have it i can let you know how good it is

xx
hope this helps xxx