I got some wax foundation and melted it (not too hot). I then used a roller like this to apply it to the unwaxed plastic foundation.

https://www.amazon.com/VORCOOL-Rollers-Painting-Brushes-Painters/dp/B07F7XLGZM/ref=sr_1_39?keywords=paint+roll+brush&qid=1556551100&s=gateway&sr=8-39