After you jerry-rig your coaxial connectors in whatever way you choose you want to use a product called Coax Seal which is available from Amazon here. Even though this product presents itself like a roll of thick black electrical tape it is not necessarily used like that. You wrap up the connection to be sealed as best you can and then using your thumbs and fingers you mold the seal into and around the unusual places and form something tight.
I have used this product with a whole slew of different types of situations and some worse configurations (for sealing) than shown in your photo. The key though is applying enough of the seal and then molding it to fit the piece being sealed as well as to satisfy your fitness to the purpose.
Long lasting. I use this seal on coaxial connectors that run to my mobile antenna and under the body of my truck and the seal was just as effective after 10 years as it was the day I applied it.
One thing -- it is a bear to take off unless you use some thinners and other solvents to help dissolve it.