Running HDMI cables behind plaster with < 25mm clearance

[Adam]

So here is the chase for my wall: http://i.imgur.com/s4LaCDg.jpg

The issue I have is there is < 25mm clearance. The wall is an exterior wall with dot and dab. I kept hitting the dot and dab so decided to chase that channel out.

My original plan was to use some galvanised steel capping (http://www.screwfix.com/p/tower-galvanised-steel-channel-37mm-x-2m-20m-pack-of-10/66834) so I can both protect the HDMI cables and potentially add/replace cables in the future if I need to.

My issue is as follows:

The backbox (which I posted here and I'm not that happy with: http://forums.overclockers.co.uk/showthread.php?t=18713949) doesn't allow the HDMI cable enough room when the capping is in place. The capping isn't deep enough to for the HDMI cable to come in to the backbox.

Given the depth I have to work with and hopefully avoiding chasing in to the brick (mainly due to the mess it makes), what other options do I have?

 

[Adam]

Thanks for the suggestions. I've decided I have no option but to chase out the brick and install some pvc conduit rather than capping. I think 25mm deep x 40mm wide should be sufficient and while I'm at it I'll drill out some more depth for the backbox and go for 47mm. This was suggested on another forum and I really think this is my best option to make sure it's future proof. Annoying as it will take more time and make a load of mess, but would rather do it now than when it's plastered and papered.