If you are hopefully using the tiny lock washer, you can hand tighten it enough to just hold the head of the screw with your forefinger since a standard screw driver won't fit. I use a nut driver on the inside of the shell and that gets it plenty tight. Once the lock washer is squeezed down it helps prevent the screw from spinning and all you need is a little additional pressure from your forefinger while tightening the nut from the inside. Works for me...

I'm guessing you're having trouble because the adj. knob and slider the strings are tied to are in the way. If this is not the case, apologies ahead of time. If it is the case, simply unscrew your tension adj. knob all the way so the slider comes off. This will give you access to the bottom screw from the outside and the adj. knob will swivel to the left or right allowing you access to the top screw. Tighten them up and reassemble the slider to the tension adjuster. Hope that helps.

Thanks so much for replies. Got it into place now. Good advice, all.

Take care.

Stephen