Generally the rear sandbag won't be very good for springers, cuz you want the rifle to recoil and vibrate naturally, and the rear sandbag won't let it do that. Now if all you are going to do is shoot off a rest, then it is OK to use a rear sandbag. Switch to SSP, CO2 or CA/PCP the the rear bag is OK to use.
As for the rest itself. All it needs to be is STEADY. If a plastic rest w a sandbag is steady, then that is OK. You can always put lead shot into the front sandbag, and that will add mass to the front rest to steady it up.