Huber's Zip system has an optional liquid flash that is a great product. But it is also expensive and you need to buy an entire contractor's tube at a minimum. What is a liquid flash that works well on wood and house wrap paper that can be purchased in small quantities and is cheap?

The background for this is we installed some large redwood planters by applying some simple home building principles to the installation. We simulated a concrete foundation by placing some concrete pavers, above ground level, on top of an adequately built base, to keep the wood bottom of the planter off the ground. We lined the inside of the planter with Tyvek HouseWrap paper, then sealed the penetrations with Tyvek tape. We tied the planter down to earth by sinking t-posts and screwing those into the planter inside, then tape sealing the entire post and its penetrations. We flashed the bottom of the planter with Gorilla sealer tape. We screwed in a trim board on top, over the house wrap paper, but unfortunately, we forgot to flash the top of the house wrap. As shown in the photo of the insider corner here, there is a place where water will get in behind the house wrap paper. Just to make this a more complete effort, I want to put some liquid flash into those corners, to waterproof another point of entry for the water. We will probably use the flash to seal the penetrations to the trim board as well.