Making panniers out of canvas map bags -- how to seal after the build?
I assuming you are going to put some washers on the inside of the bug so the bolt/net holds well.

I would expect silicone sealant under the washers (on the inside of the bug) and the clips combined with a rubber washers between the head of the bolts and clip and the inside washer and net would do the job.


End of bolt
Rubber Washer
Pannier clip
Silicone Sealant
Bag
Silicon Sealant
Metal Washer
Rubber Washer
Net


However after saying the above, as the clips will be on the inside of the bugs, you will have to be in very strong rain before they leak, you may well find you have other leaks.   A simple small waterproof rucksack liner inside of the bag may be the best option.