Two 90 x 60 council slabs (Shenley Lodge MK5)

Does anyone have two 90 x 60 x 5 council grey slabs in the MKish area? I'm helping to build a DIY base for an elderly friend. Someone gifted them an oval shaped water barrel and it needs raising up to use the tap. Anything structurally sound will be great even if corners are chipped but the smaller 60 x 60s size would be unsafe for this. Thank you for reading this post.