Nitro,
If you are going to build one I would suggest that it is made adjustable for height, and also for elbow and/or shoulder rests.
I've shot doubles from standing rests that were made for someone elses height and found them to be awkward and difficult to hold steady from.
Too low or too high can make a big difference to how steady you can hold and also how the recoil will feel with the big stuff.
When I lived in outback Queensland I had a very basic but very good double rifle rest on my front porch.
There was an aluminium awning over the porch supported by several 2 inch square aluminium posts.
I would fit a sliding carpenters clamp to one post that was not far from a wall on the house.
The clamp could easily be positioned up or down for precise height and I would wrap a towell around it for padding.
I would use this as the rest for my forehand and I found that I could lean against the house wall slightly with the back of my left shoulder.
This position was as steady as shooting from a benchrest and was very good for sorting out double loads and sighting etc.
It was also very kind in the recoil department and was as comfortable as shooting offhand.
There was a dam bank at about 90 yards where I would set up the targets.
Lord how I wish I still had it, especially when working up loads for my 9 1/4 pound 470 double!