*** Hi ANthony
fix a series of horizontal card strips between the Auhagen and ply portal, say 15mm wide and 10mm apart, covering the whole arc... let dry thoroughly. It can fix to the Auhagen facing strip and to the inside face of the arc of the ply.
any good firm card will be fine - it doesn't have to be super strong, but glue it well. Pinning while it dries is fine, it'll not be seen when finished anyway.
Cut the brick paper so its a neat fit, then wrap it firmly face inwards around a can or bottle about the same size as the portal. Use a damp but not wet chux cloth or similar disposable kitchen cloth and wrap around it to hold it to the can/bottle shape. fix with rubber bands etc....
Now, keeping fingers clear, hold over a steaming kettle for a couple of minutes then put aside to dry in its own time.... overnight is good.
The damp cloth will slightly soften the card and the steam will help it relax and take on the new shape.... Slowly drying it will let it fix its shape permanently so it holds it when the cloth is removed and makes it much easier to glue inside the tunnel.
Now cover those card strips with adhesive and fix the brick paper to them. The curve of the card + the card strips will make a stiff and strong combination.
You could use any contact glue, PVA or perhaps even thin double sided tape.
When dry, add a bit of insurance by adding a layer of glued paper over the outside which covers the strips and the card back at the same time, this will stop the brick paper falling away at any time.
once all this is dry, is time to turn that brown card into dark gray/black sooty brick!!
Richard