You do right to get some guarding sorted and the reality is finding someone who is willing to do it will be extremely difficult.
Several schools of thought exist on guarding, but the facts as I see it are you can't touch the existing Cab structure in any way, and any guarding must be accepted as not being approved in any way apart from the fact you are taking all sensible precautions to minimise the risk.
However, having it designed/constructed by an industry recognised specialist with the correct fabrication knowledge and qualified welders etc clearly demonstrates you have taken all reasonable precautions.
Sometimes factory guarding is available, and in limited instances approved third party guards, but these are mainly for Demolition and not necessarily suitable for the task.
Changing windows out for Marguard especially where Mulchers or Harvesting Heads are concerned is a must, and in reality can be a lot easier than some would make out, and can be done skilfully so you really wouldn't know it's fitted.
Your comments on nothing stopping anything from above lead me why my current Kubota is guarded as it is. It may not be the limb you're shearing that gets you, it's most likely that bit of dead top that you've just dislodged that's hurtling down towards the roof without you knowing!
I had this happen whilst felling with the grab of my old Kubota, and it's not something I'd want to repeat. I was inches from a top window unwanted visitor!
The top of my machine will now keep me a lot safer than before, and the times the whole structure rings like a bell, lets you know that it's money well spent.
The front window is obviously maximum risk, and for those daft enough to shear without a guard, at least take the levers off your track pedals.
Why? If it all goes wrong and that limb comes in, depending on which way around you are, you may have the machine back away from it, or worse keep tracking that limb into the cab with you unable to do very little about it!
Personally I'd like to sit down with the HSE and take a look at sensible precautions for a minimum standard of guarding for Excavators in Tree Work, before we read of a tragedy with the inevitable knee jerk reaction that would follow with either bans on certain attachments or machines looking like tanks with a periscope to look out of!
There are some great companies that will do the guarding, Forest machine Services, Wilsons, Caledonian Forestry, JCC Engcon are the ones I can think of but it would be great to hear of more.
Cab Care and Jayscreen do a range of guards for machines that are approved for demolition.
My final point really has to be, how does anyone use a Shear on an Excavator without additional Front/Top guarding and get it past the most basic Risk Assesment, and the same would apply to a screen guard with a free swinging log grab?
Eddie.