Some of us are fortunate enough to have stocks of the clip and bar fittings. There used to be a framing shop in Totnes that did all the old fashioned stuff like this and they were called Western Arts, they used to make up stuff like this on a regular basis, but eventually the owner closed down and the building was redeveloped over perhaps a year. I bought just about everything in the place, except their Euro "butterfly" underpinner.
So I know what they used because I've got boxes of the clips and bars. Like you, I want to make quality strut backs, but getting decent oak boards is a significant problem and sadly, if you are going to buy boards made from well matured oak, the price is going to make it hard to get a decent profit margin out of doing this. I not saying it can't be done, but you have got to find a reasonably priced way of getting the oak for the boards.
As for the screws, this is a definite problem area. The holes in the clips and bars were never intended to be suitable for fixing with wood screws. I would not even bother trying to fix these with wood screws, getting enough grip in the wood with screws tiny enough to go through the holes in the clips and bars, is simply not going to happen. You have to have a re-think.
Back in the late 60's when I left school and became an apprentice, the country had not at this time switched to metric screw threads and a popular version that existed then was BA size threads. BA stood for British Association, the threads were quite fine and the smallest screw thread sizes were tiny, but threads were easily tapped into all manner of materials. These threads are still used by those who make models and model shops still sell BA nuts, screws, washers, plus taps and dies.
There's not much material around the holes in the bars, but I fairly sure that there is enough to tap the hole with a suitable sized BA thread. Unfortunately you've not got much choice, but to used a screw with a countersunk head for fixing to backing boards and countersunk heads are the strongest, so I think that the wood for the backing board need to be think enough the glue the screw into a good thickness of backing using Araldite, so that nothing is going to move, or work loose.
I would also use a pan head screw to fix the clip to the strut and I would drill and tap a suitable size threaded hole in the wood of the strut and glue the screw into the wood again using Araldite. I have not seen any evidence of others doing this, nor have I done this myself, but I think that this is the way that I would tackle this problem.
http://www.squirestools.com/files/12-06x.pdf
Squires is not the only model and tool shop around, but they just happen to be the one that I know. Other suppliers in the same market are likely to be just as suitable, so shop around.
What are the clip and bar fittings?
