"Riding the Chair" - Ice Timer & Attachment Help

I could use some guidance on two points:

1. I’ve never used an ice timer release before! What should I use to freeze the water, and how should I get the ice out of that container once it is solid???
2. How can I securely attach the dildo / butt plug to the chair? I’ve tested (without bondage) and any method I use to secure the dildo to the chair isn’t strong enough to stop me from pushing it away.