Installing wonderboard below acrylic shower pan

I'm replacing a cracked shower pan. The floor is structural planks with 1/4" between each plank. To provide a solid base for spreading the mortar beneath the shower pan I thought to use wonderboard. I tiled the bathroom floor up to the old shower pan and used wonderboard underneath the tile. The new showerpan is smaller than the old one, so in using wonderboard under it, the level is the same as the wonderboard under the tile so should be easy to tile up to the new pan. Question though, what I've read suggests thinset mortar under the wonderboard. This may be difficult to do with the gaps between the planks. Any suggestions would be greatly appreciated. Thank you.

 

Put down a piece for 6 mil plastic, then the mortar. You don't need to stick the wonderboard down, just fill in gaps.

 

Thanks for the quick reply. I have a greater problem. The acrylic pan doesn't fit the drain, so I will need to replace with one that does. It was part of a shower kit. I'm keeping the glass surround, but returning the pan and abs glue up walls. I've tiled another bathroom and have decided to tile the shower walls. But I've already put up green-board. Can I put backer board over that for the tile, or chalk it up to a learning experience and remove the green board and only put up backer board?

 

Remove the greenboard, install vapor barrier and then the cement board.

 

Thanks for the quick reply. I was afraid of that. Luckily not too expensive a lesson.