I applaud...
...your can do attitude...improvise, adapt, and overcome, but I am not so sure that sealing up that "hole" is wise...I am sure that the tail light assembly is anything but waterproof, and now what I think you have is a closed space that is ripe for collection of water...hence...rust, or worse. I believe those "holes" are their for a reason, and perhaps a modification of your adaptation with, say, small drain holes, may improve your design...just my dos centavos.
...your can do attitude...improvise, adapt, and overcome, but I am not so sure that sealing up that "hole" is wise...I am sure that the tail light assembly is anything but waterproof, and now what I think you have is a closed space that is ripe for collection of water...hence...rust, or worse. I believe those "holes" are their for a reason, and perhaps a modification of your adaptation with, say, small drain holes, may improve your design...just my dos centavos.