The screws are a special type and I don't think you can find better ones. They are specially designed to screw into plastic.
The question is if you have stripped the threads of your board or if they are just undoing themselves slightly? I have heard of stripped threads, but that is almost always something that happens the first time you mount the straps. I haven't heard of problems with the straps unscrewing themselves.
Maybe you can reinforce the threads with the thread compound that is recommended in the footstrap section under Productc>Product assistance>repair guides.
If the threads are intact, another idea would be to put a drop of loctite between the bolt head and the washer (I don't know how the loctite would react with the plastic in the plug, so I would not put it in there).