Hi Brian,
I can't give you any particular advice on making a hide, other than to say, unless you want to use very basic materials to keep the price rock bottom (and why not!), you can buy portable hides for a few tens of pounds. I seem to recall warehouseexpress adverts showing them, so it might be worth checking out the link at the top of this page.
However I am slightly puzzled by you request.
Obviously I have now idea of the layout of you house and garden, but would you be that much closer to the birds than you are looking out of your (kitchen?) window?
Indoors, you can at least allow yourself the luxury of the occasional cough, and cup of tea, without alarming your birds.
If you really do want to pit a "hide" beneath your tree, may suggest an alternative of a screen. Probably doesn't need to be anything special, or any particular colour and such, just so long as it screens you from the birds, without screening the birds from their feeders.
Again, it will cost a bit of money, but a lot of nature reserve use willow screens, and I dare say you could get something like from a garden centre.