We have broken out basement floor slab to install a drain for a new shower. Trench is approx 6 feet long by about 6 inches wide (irregular not saw cut). I know that a vapour barrier has to be reinstalled but I'm not comfortable installing 6 mil poly due to the difficulty installing in a narrow irregular cut trench.

What I'm looking for is a liquid or spray applied waterproofing product to apply on top of the gravel in the trench to form an effective vapour barrier. Any ideas?

These folks may have an opinion on use of their product or another suggestion:
SPRAY-ON VAPOUR BARRIER FOR CONCRETE

I don't follow why it is so difficult to place the 6 mil poly. Possibly use a width of poly that is as wide as the widest opening and trim it in the bottom of the opening with a good razor knife? Spray some foam insulation at the joint between old and new poly for total seal if you think it is necessary.