Wheels Manufacturing makes an aluminum insert to take up the space between the rim and valve. It is called a "presta valve stem saver". I used them for a switch from schrader to presta a long time ago. Just be sure to keep track of the spacer when you change or patch a tube on the road.
$5.26 with free shipping on ebay https://www.ebay.com/itm/WHEELS-MANU...SgdQ:rk:1:pf:0
$6 on Amazon https://www.amazon.com/Wheels-Manufa...66451793&psc=1