How to TimeMachine backup to firewire disk with secure sparsebundle

Hi. I'm on the road a lot and I'd like to be able to carry a backup with my. I understand that Time Machine backups up are not secure, so I've tried to create a secure image to backup to, but I can't get it to work.

1. I created a password-protected sparsebundle image on a firewire/USB drive calle "road_backup.sparseimage"

2. I've confirmed that I can write/read to this area and that it is password protected.

Problem is that when I go to Time Machine preferences and select the backup disk it only gives me the option of the firewire/USB disk - not the secure image.

I there any way to do what I want to do - either by password protecting the whole disk or making Time Machine write to the sparsebunde?

In know that I can vault the whole disk - but I'd rather not do this.

OS is 10.6.8

Macbook Pro 13"

Intel

Thanks in advance for any help that you can give me.

49.