So i did not go for de forth hole solution . The bracket has 3 hollow spaces and is cast steel .
I think that drilling wil weaken the bracket as cast steel can be a bit brittle !!
Instead i filled up the spaces with milliput and put a washer under the bracket at the lowest hole .
From the inside i took the rubber under the plates away (the fiberglass layer will protect the inside sufficiently)
So now the lowest part of the bracket is sandwitched between 1 washer on the outside and the bracket plate (with 2 of the same size washers )on the inside .
The outside bracket plate rubber i've put back on, too and forms quite nice around the washer . Then i tightend the bols with a little each until the where tight and put a little super gleu on them . (i always do that ; the bolts stay put and if you want to unscrew them the supergleu just brakes off )
There is no movement space for the wood anymore and i hope it will work .

Ive changed my plan .See my post lower
@ leedy
I think your brackets were not made of cast steel .
So drilling them is no problem .
@ Olsongo
Wy you asked of the conga's had a fiberglas lining ?