I have done the procedure below on different boards before, not a BX6 r2. BUT, I'm looking at a BX6 r2 as I type this to direct you through it.
1. Obtain two thin skinny zip ties. You'll also need either the broken top clip, or a similar piece of metal to fullfill the connection.
2. Insert the battery. It will slant upwards as it has nothing to hold it down.
3. Raise the board up and look under the battery housing. Your view is from the left of the IDE connectors/SDRAM slots. You'll note there is clearance under the battery housing and it touches the PCB on the motherboard in 5 places.
4. Run the zip tie UNDER the battery housing toward the first ISA slot. You are making a loop. WARNING... don't make the loop very tight as you may break or distort the battery housing. Or you can use two zip ties and run them from front to back.
This should work. I've done this about 5 times on various boards and computers. It's somewhat better than electrical tape.
Hope this helps.