Two eye bolts screwed to the floor. The chain on the cuffs runs through one eye bolt with the hands cuffed in a position that is between the legs of the sitting captive. The ankles are then cuffed to the other eye bolt with the legs extended.
I think it would be impossible to unscrew the bolt with the chain of the cuffs running through the eye bolts, when the hands and ankles are attached at different points.
Then what would I do with myself? 
Sorry, not trying to pick on you, it just sort of fell into place.
I’ll second this one. I’d use a load-bearing wall where carpenters are less likely to take shortcuts. Drywall is easy enough to break that it’s simple to set up and virtually all buildings use it. Just make sure there’s no rot due to moisture, termites, etc.
Since he’s a cop, how about those police nylon zip ties instead of handcuffs? He zip ties her hands and feet, then zip ties her cuffed feet to a sturdy pipe.
She waits for the sun to hit the window just right, then manages to fashion a sort of magnifying device out of her glasses to burn the zip tie on her hands off.
Let’s turn to the experts for advice.
Or check out Stephen King’s Gerald’s Game - even though you said no furniture, the damsel in distress is handcuffed to a heavy bedframe and is alone (after a sex game gone wrong). She manages to
free herself in the end with the lubrication of the blood from the chafing of her wrists.
Zsofia you should rent the movie 9 to 5. An oldie but a goodie.
Three women kidnap their boss and have to keep him restrained for a few weeks. Of course they’re checking on him every day too.
It’s been too long for me to describe from memory but you should see the rig they use to keep him restrained but give him some freedom of movement. It took more than a little prep work though.
Do you use LP Gas for water heaters in rural wherever? Gas pipe is usually quite thick and sturdy. If forcibly yanked out of the water heater, a spark saved for later could help make an interesting escape. The wiring and conduit for an electric water heater might also have shocking results.
Being a cop, he knows he wants it secure, but also not suspicious looking if someone happens upon his cabin on all those days that he isn’t using it.
Interesting to be cuffed around the pipe, unscrew it and have the gas burn ferociously. Good distraction for the escape.
I love M&Ms.
My primary chimney goes right through my attic. If I were to keep someone up there with no chance of escape, I’d just run a heavy chain around the chimney and connect it to a steel collar around the neck. Just to be sure, I’d also suspend the person by their ankles.
I’d drill a hole just outside of a door frame, thread the handcuffs through the hole, and then handcuff her there so each hand is on one side of the wall. The door beams should be sturdy enough to prevent escape.
This could actually help with the escape part. She’s got her hands cuffed with the chain looped around one of the feet of the tub, at the end away from the drain. It’s full of water. Eventually she manages to contort herself up so that she can get her leg into the tub and just barely reach the stopper with her toe. The tub drains, but is still pretty heavy. With great effort, she manages to briefly lift it just far enough to slip the chain under the foot, and she’s free (but still cuffed).
ETA: You could even add that the tub drain is clogged and takes hours to actually drain. 
A large eybolt or chain strung over a rafter would blend nicely, you just claim you hang up your hunting kills to clean them. Would also easily explain large volumes of bloody mess at that spot.
Another idea using eyebolts:
If you want to avoid them unscrewing it, I would place the bolt far enough away that they couldn’t reach it. Let’s say the handcuffs allow for a maximum reach of 6". If the eyebolt is placed into a beam 6" thick, the bolt on the other side of the beam would be too far away to reach.
Also Black Snake Moan
Water heater. Yeah, it’s heavy. It’s full of water.
There is also a knob at the bottom to drain it… Or at least a pipe stub to break off, being heavily corroded. The drain may or may not lead outside. If not, the floor is ankle deep in water. Electrocution of bad guy follows as she stands on the toilet, safe. Of course, she slips and now has amnesia…
Its not much fun,but handcuffed,around the base of a toilet,“hugging” it & told to stay reclined…my mistress,uses this for one of her punishments,when i’ve been bad…
Not for nothing, but I think Zsofia’s finished her story by now…
Refrigerator door handle? This method would certainly slow down an escape but allow the character to overcome some challenges in order to eventually get free.