Try the good old Boxer favourite that people can't understand......
Prayer mats :D
You can get this anti slip matting from a variety of different places even the pound shop
do it. B&Q and some other DIY stores do it.
So you might be able to find a piece that blends in with the green carpet.
I have the black anti slip matting which works for my dogs. It's cut in the same shape
as a beer mat but bigger, you should be able to work out how big you need to make it
for your dogs rear feet to fit in stand stance.
Plus you should be able to practise before Crufts so that your dog is used to standing on it.
Unfortunately I didn't have the presence of mind to take it (anti-slip matting) in the show ring
yesterday at an open show and some rather unsporting BOB winners in the working group
refused to move round (despite being asked) on the unoccupied matting to let the other dogs have some matting too.
So although two sides of the ring had matting, the other two sides didn't.
So consequently my girl ended up doing the splits on the slippery floor when trying to stand

Not surprisingly we were unplaced in the group.
Hope you manage to find some and Best of Luck at Crufts :)