go for 10kg as weight rarely makes a difference unless it's way over 20kg or it's a really oversize parcel
depends if the splitter is the full length of the car? l x w x h try, 30 x 150 x 30 if not to 30 x 200 x 30 if it is, it certainly won't be any bigger than that
30 x 200 x 30 @ 10kg = £11.85 + VAT, you could adjust the figures afterwards when you know the correct dimensions