You can use a simple cube model entity - flatten it if you like so the player can stand on it. Give it a transparent texture. i.e. you can make the whole texture black if you like + a 100% black alpha channel. In the entity script set the TSP like so :
transparency =2
That should do it for your entity = invisible.
Use a copy of the teleporter entities script (IN/OUT) files (renamed) if you like or make up your own using the move player postion and player proximity command fucntions for your main functions in script.
Using a modified where necesary copy of the teleporter script files you wont go far wrong + your two new invisible teleporter entities.
Thats it.