the front of your dashboard just pulls off. you can see the gap where it attaches about 1/2 inch in from the dash. the whole piece around the heater vents, radio, climate contol panel, etc. i am assuming that you already had that part off the dash.
once you remove that part you can use some sheet metal or light weight steel to fabricate your own bracket. i can't tell you how to shape the bracket because mounting location will dictate that. anyway, bend the bracket appropriatly and mount to the inner part of the dash but have the bracket shaped in a way that you can reinstall the front of the dash.
that being stated you can mount the speedo anywhere
best part about it is that when you remove the speedo and bracket know one will ever know it was there, no screw holes or anything...