Clear Epoxy as normally used for lamination work (2:1 mix) is already pretty fluid, close to water.
Doing the operation in vacuum is a very good idea, additionally heating the whole assembly after filling makes the epoxy thinner. 180 deg C is optimal, but probably not suitable here. But if you can heat the assembly to 80...100 deg C, it makes a big difference. It also accelerates curing time significantly, like from 24h to 4h. At least heat to 60 deg C (curing 10h).