Hope this helps. The chase is angled (externally - top and bottom only) so that it slides in and sits against the bed of the press.
The inside is square obviously so that you can lock up your type.
You should be able to see the chase rests at the bottom of the bed and there should be a spring loaded clip at the top to hold the chase in place.
I've been told there are quite a few variations of the Model Presses as they kept developing and improving them....so yours might be different! might be worth mocking one up in timber before hand. Hope you get it up and running again!