Jon, sorry, not that I know of. I can buy fibreglass bulb keels to fit my bigger Thames Barges, but they would not fit a pilot cutter. For my smaller barges I make what is essentially a shallow box about 3" wide, 10" long and about 1/2" thick. A 6mm threaded rod goes through the top for bolting to the hull. Make the box but do not fit the front. Fill with either sheet lead or lead fishing ballast, put side on glued and screwed. The leading and trailing edge can be rounded off. I have used brass sheet or ply for the front and back, depends how much weight you want. Probably would look ugly on a pilot cutter, but it works. Hope you can understand what I mean.
As I use this it is a fin keel,but there is no reason why you could not adapt it to fit along the long keel of your boat. As an afterthought, Mistyoptic made an extension weighted keel much the same idea as this for a small sailing boat he built in the Dutch style recently. Find the posts on that, he did do some photos, they were in Build Blogs under Dutch Auxiliary