You'll probably find it best to leave the screw off and wedge the channel in using nylon ties. you can also use the nylon ties for end clips, somewhere around 10 centimetres in length. What channel are you using, I found the brass traditional OK but heavy, I now use Pulex aluminium, they are very good.
I find the easiest way is to put the tie on the rubber about 2mm from the end and then slide them in together. Sometimes it's easy and sometimes not! You can put the tie above or below the rubber, use a bit of washing up liquid if you want. The best result I've had wedging the channels is to slide the handle up onto the channel while holding the tie in place. Use some washing up liquid to lubricate. As you move the handle further up the channel it will tighten, so you may have to tap the tie lightly with a piece of wood. The angle of a channel in a Wagtail is different to ordinary squeegees and is more crucial. I find the best place for the tie is directly opposite the opening.
Some day.
It stays. Marine epoxy.
