Rob,
They are not adjustable, i guess it depends if the ones you had from mal are original, repro from a supplier or he made them himself. The straps are kind of like the soft tops.. unless you install them just right,, they wont fit just right. In the photos yours didnt look to have any tension in the strap.. they were hanging, if they are tought they will support the cover from underneath, if the part with the tennax fasener is a little shorter it pulls the top back towards the rear of the car, this combined holds it nicely in place. As i said, if they have stretched over time, or are sagging they will not hold it as firmly.. you could perhaps take the hooks out and place them a little further down to give them some more tension.