I agree with Evan. Although the compass was within the gimbal, and on the mounting bracket, it was also recessed partly into the cut-out in the instrument panel.
The bracket you've made can be retained, and a new compass made as described by Evan, or alternatively, use a piece of plastic tube, filled with plastic rod but leaving a hollow, which can be painted and then filled in with a drop of PVA. This will dry clear, and look like glass. Note, however, that generally these instruments had a non-reflective glass, so that they could be read at an angle, and very often, when close to the real thing, it's difficult to tell if there's glass there or not!
You've done a great job on it, but it does look far too large and, by the time the pilot's seat and control column are fitted, it'll fill the space between, and could cause interference problems.
Below are a couple of pages from the Mushroom book on the Walrus and Stranraer, showing the cockpit and the compass, and I think you'll see the difference here.
The book is 39 MB, so too big to e-mail, but I can put it on CD an send it on the next passing pigeon, if you can wait that long, but I'll need a mailing address via a PM.