You need a fuse which is safely below the peak of the pack, so around 100a for the battery Mario posted, though the lowest you think you can get away with is obviously advisable.

Such a fuse can be found on Technobots but is probably cheaper on Ebay. Something like this, you might save a a few pennies if you search deeper: http://www.ebay.co.uk/itm/2-X-100-AMP-1 ... 3f1b0ee5af

And a fuse holder, for example: http://www.ebay.co.uk/itm/Maxi-Blade-Fu ... 2ec1bce2c5

I'd also suggest a Lipo Alarm, though I don't think that's in the rules.

You need to mount it securely, though with shock absorption, and with at least 2mm of metal between it and the arena floor.