To be fair you are asking us to advise on :
1. a setup we can not see
2. a diy tray project
Any recognised person on this forum can not possibly advise you as it is outside of good practice.
To help you I would sugest you forget a shower trap and get a wet room gulley / drain from a recognised manufacturer. While researching which drain you can browse through their installation drawings and get a better idea of how they are fixed in-situ........
Hope this goes at lease some way to helping you.
cheers
tiler