I came up with a base that works very well for my wife's suction cup dildos and wanted to pass it along...a 25 pound or a 15 kg barbell weight plate that has a flat side.
The suction cup sticks very firmly to the flat metal surface, the plate is heavy enough that it doesn't move or lift at all and the diameter is small enough that it can easily be straddled while on your knees. And, maybe best of all, you can use it on your bed so you have a nice comfy surface for your knees!