I got tired of pushing epoxy in to the slots before the epoxy dried so I got these applicators that greatly speed up the process. The 18 and 20g needles are the ones you want. 3mil syringe is about right.
https://www.amazon.ca/gp/aw/d/B07YB8SYCG?psc=1&ref=ppx_pop_mob_b_asin_title

AliExpress have a much bigger selection and better prices if  you're not in a rush.