There's some stuff called "Seal-All" -- get it at Home Despot, probably Wally World, for $3 or so for a small tube. It works like a contact cement, but if you let it dry all the way, it also is a seal-coating. It dries clear, or maybe a little yellowish.
The only shortcoming is that it's NOT UV-resistant, so if it's exposed to sun, it will harden and crack.
If the top's black, another option is "Liquid Rubber" -- same stores, plus car-parts places. It's a black neoprene paste, about as thick as vaseline, dries matte black. I've used it to patch pinholes in camera bellows. I believe it IS UV-resistant.
BTW, the seam-sealer I used on my tent last year dried clear in thin coatings, but whereever there was a drip, it dried whitish.