dog flap?

I am wanting to put a dog flap in my son's bedroom door. This will give him more privacy instead of leaving the door open all the time as he cannot open or close the door himself due to his illness.
I've looked online and all the dog flaps are for external doors, my internal doors will have sections that are hollow inside?
Any suggestions please "Thanks"

Insert blocks of wood where the void is between the two panels Sue thus making a frame to fix to

Thanks Ian - That makes sense -
I'll tell husband to take the door of first and may be add glue inside the panels before inserting wood!
"I will be the foreman keeping an eye on him" lol

... I'm sure it'll work just fine with a mug of tea mid way through

Dear suzieJ, May I kindly ask you the height at which you have installed the flap door when measuring from the bottom. I am about to install one on my door but my Belle is still a puppy and I do not want to make a short flap for her.

Hi ritman
My husband put the rise at 51mm.
I when on here to check out the measurements- http://www.modernpetdoors.com.au/sizes/

Sue, does the flap work ok ?

Hi Ian,
The door flap is great, no need to lay infront of the door whimpering to let me know they want to check on their master.  Eveyone is happy- son, dogs and me for not having to go upstairs all the time so they can get their cuddles

Great post

Great news Sue... Anything for an easier life

Fully agree, my adult son is always happy to see his/our dogs and they always great him with lots of licks if he is awake, and other times seem to sense when he is feeling unwell when they curl up at his side of him.
Its lovely to see