Agree with Glenn and Ruce here. I reckon a 3" flat bottom forend would be a PITA in the ute. Unless you set up your Navara with hydro-variable self levelling suspension to ensure your rest is perfectly level. Yeah, nah.......
So with the long range BR you are stationary. Doesn't matter how much junk you have hanging off the stock? Are there any particular weight limits on the set up? I reckon If you are looking for an all rounder I would be going with a forend design than best caters for your intended field use but with additions can be utilised for the BR work.
Does your field use include the use of a bipod? If you intend to use a Harris or similar then this sets a limit as to the forend width. Harris bipods fit the forend cross section on my Rem varmint perfectly, whereas the two Sako varmints require the use of a wide forend adaptor. While you could use an F-class style bipod that mounts onto a rail inletted into the underside of the forend they usually don't have folding legs so are a pain to manoeuvre in the field or in and out of the ute. Better for setting up in one spot for a while.
So the following assumes the work will be done by RAF industries. For your BR work you could inlet some female threaded thingos (ferrules?) into the underside of the forend that allow you to bolt up a piece of polished alloy C section to provide a straight and level surface to ride your front bag.
For your butt you could create a horizontal bottom edge in a variety of ways your could add wood to the bottom edge, or you could use some alloy of steel to create a horizontal strip. Inlet some threaded ferrules into the end of your butt, then you could either run an L-shaped piece of alloy or steel that forms the butt plate then runs forward under the butt towards the pistol grip. For field use screw this off and bolt up a rubber butt pad.
So there's some ideas for you. Your BR stock might look a bit wild but for a RAF industries Zastava shooter I don't think that would be an issue
Marcus