I am redoing an Ender 3 Pro and added a silicone hed heater to it. I did not like moving the wiring from the center to the original wire holder so this was quickly designed.  

Clean the silicone pad with alcohol where the top part will go. Lightly sand to scuff the surface then coat the top part mount area with Red high temperature RTV or other high temperature adhesive and clamp it to the bed. Wait about 24 hours before you do any more work on that part After the clamps were removed. I added a bead of RTV around the part to add additional strength.  

Bottom part uses 2 M3x8mm screws with T-nuts to hold the part to the Printer and 4 M3x8mm screws with nuts to hold the 10x10 cable chain mounts in place on the top and bottom. Printer is now in service and this and the cable chain are working well.

I printed the top part  with ASA. PC or ABS would also work well. I would avoid PLA  as it will sag. PETG might work if all you ever print on the printer is PLA.  Parts are small so should be printable even on the Ender if you have a all metal hotend.