I had a vintage canvas striped awning in green and cream made which came with the smaller welt for my 2012 177 that has the larger rail.
Tried the slit tubing fix, it was either not done right by me or just not workable, it would jam going into the rail or trying to get it out, or the welt would come free on any sagging of the awning, you are on a stepladder and it is unwieldy, not practical for me.
I then tried small sections of slit tubing over the welt, popped on as I fed it in, not great result, the awning would come loose in stronger breezes. Last year I just gave up and used a cheap little lounge chair umbrella for shade and put the awning fix on my "think about it" list.
I have heard of people using pliers to squeeze the channel, but I am hesitant, so I'm going to use some 5/8 rigid piping in small sections with loops of paracord that I'll run through small holes drilled behind the bead into the flat of the welt. In other words give up on trying to hold the small welt in the channel and use tie points to sliding anchors in the channel.
Essentially the make your own awning grommet attachment idea (several small tube attachments for paracord loops to run in the rail) modified for my awning. There will be a small gap, although as I will run the paracord loops from the underside of the awning welt, it might pull the welt pretty tight to the top of the rail. Originally I was looking for aluminum tubing but the stiff plastic Ptex style is sold at the hardware store for all of about $2 for 4 feet and will be much easier to cut and no sanding or jagged metal edges.