NFS is one of thelargest security holes in *nix systems... I think you would be better off asking this in a linux forum where some pros could help you lock down NFS since you claim that you require it.
Do you need only locall access.. then you can try the following:
hosts.allow
#
portmap : localhost 192.168.1.0/255.255.255.0
rpc.mountd : localhost 192.168.1.0/255.255.255.0
#
#end
to allow selected machines to access:
exports
# Allow only those ( 3 ) machines ( *.1, *.2, *.3 ) can rw, others are readonly
#
/opt/home 192.168.1.0/30 (rw,no_root_squash)
/opt/home 192.168.1.0 (ro)
#
# end
Sorry... can't help you more than this, I am not really a *nix guru, only play around for fun once in a while.
When in doubt, deny all terms and defnitions.