Well....this is what I have done so far. I cut and sanded a piece of 2x4 to a few thousandths less than 3", leaving the sides at a right angle to the bottom, and then I took all of the side pressure off of the bag, and worked the block down in till it bottomed. Then I put all the side pressure on the block and let it sit for a while. After that, I backed off the side tension, removed the block, and examined the bag, which was starting to take shape pretty well. Then I dipped the tip of my index finger into a large pill bottle of HBN that I had bought some time back to try on my leather bags, and rubbed it into the new leather all over the inside surfaces where the stock might make contact, reinserted the block, tightened the side tension back to max, and I am going to leave it that way all of the time that it is not in use. I left the bottom corners of the block as cut because I want to make some room in the bag in that area, to alleviate corner drag. The stock is very slightly radiused in that area. The reason that I did not wet it, is because I have a trip to the range planned for tomorrow, and that would probably not leave enough time for it to dry...perhaps later. The HBN seemed to make the leather nice and slick (for leather). We will see how it shoots.
When I filled the bag, I did not fill till it was tight, but rather left a little room in the tops of the ears so as to avoid having a drum tight bag after its straps were clamped down.
This particular windage top has thick pieces that clamp the bag straps and the bag had been a little low between them when first installed, so I cut a spacer out of quarter inch modelers' ply and put it under the bag. Also, I just noticed that the bag sits a little out of level on the surface where the rifle will sit. Before I cut a wedge to fix that I am going to try it out and see if simply adjusting the front legs to compensate takes care of it.
Added a little later: A few taps, from a very light hammer, on the top of the block, shifted the sand and leveled the bag.