cas04 and mgs04 are fine to use, you just won't be able to mix the cas04 in a 500ml container, it just won't dissolve, add the powders directly to your 50ltrs or RO and you'll be fine.
Sorry, my calcs where off, I did 50 gals, not liters, 4.5g of CaSO4 and 5g of MgSO4 will give you around 5.2...