That's OK.
I'd found the Penn Elconn website but the page I found had massive quantities for massive prices. That's a lot more like it, I'll get some black ordered.
In order to escape Antiques Roadshow, I did more cab building. Previously I'd got the battens on the side panels so now it was just a case of sticking the side panels onto the bottom, then putting on the top. Then see how well the front and rear panels fit.
I tried the rear panel first. It didn't fit in one way up, so I reversed it and it popped straight in. Slight gap but the rear panel is destined to be fixed in place so there will be some filler going in there.
Then the front panel - I think I'll be doing as Phil suggests and screwing this in place with 3mm thick neoprene adhesive strip 10mm wide round the joint. The fit was nice and snug.
Having done that, I went round the top and bottom panels putting the cross battens in place.
To do: cut hole for speaker, fix rear panel, make and fit cross brace, put wadding in, round over edges, fit front panel, paint, add corners, add handle, wire connectors, install speaker.