You could try barrel nipple clamps like
https://www.amazon.com/Master-Bondage-Barrel-Nipple-Clamps/dp/B005I64CXM/ref=cm_cr_arp_d_product_top?ie=UTF8
Or
https://www.amazon.com/Fetish-Fantasy-Nipple-Barrel-Clamps/dp/B00FINNV00/ref=pd_sbs_3/143-2359324-8729740?pd_rd_w=9wmxW&pf_rd_p=0f56f70f-21e6-4d11-bb4a-bcdb928a3c5a&pf_rd_r=PEVARXN65W7HJ0CGRENA&pd_rd_r=7f35a230-5923-47d5-8ae2-08eb6a38c4a9&pd_rd_wg=7p7S8&pd_rd_i=B00FINNV00&psc=1