What's the best way to seal this leak?

The side sticks out so much I think that most of the std stuff isn't going to work ... I would go with Eternabond tape.

It's likely you have significant water damage in the cab over area - that might require a rebuild for a proper fix - there are lots of threads on that subject which you can access using the forum search feature.

I have had many differnt spots of needed repairs. I always carry and use this product inside and out.
by DAP, Kwik Seal Plus, bright white.
Premium Kitchen and Bath ADHESIVE Sealant.

Have installed vents, window seals sinks and patched many spots.
NOW a cavat If you are going to put it on a wind bearing surface,you may wish to put in some small stainless screws to hold it securly. The adhesive will do a great job.
I have not used that particular product you have, but try it in a place to see if it will hold.
And always use a pressure applied to the area after you put it all togather.. Make sure it is clean, I alwaus carry a can of denatured alcohal to clean as it cleans and drys very fast.
My 2p
Good luck
Chuck in Falcon Heights, TX