Line inside with cheap value pate, shove in kibble with Frolic interspersed (crap but they go mad for it), mix in a couple more blobs of pate to make all sticky then wedge a biscuit or chew in the top.
Puppy kibble (small bite) mixed with value cream cheese to make it sticky.
For emergency quick preparation I use Primula - squirt and smear
I also have one of the Kong bones and have discovered the smallish Dentastix wedge in the ends of it perfectly - about all they're any use for but dogs go nuts for them.
My dogs favourite is that sandwich paste in a jar (all the rage in the 80! ) with puppy kibble (free samples from the local feed store) and frozen! He loves it and it keeps him occupied for a disturbingly long time
There are tonnes of recipes online! I've tried the one where you put it upside down in a mug, crack an egg into it and microwave for like 30 seconds. It worked! This one doesn't do Kongs, so I have about 4 of the things lying around covered in manky old peanut butter.
I use my dogs' kibble mixed with sardines or pilchards in tomato sauce, then plug the top with Lily's Kitchen wet food or Fish 4 Dogs Salmon Mousse. I always freeze them, as my younger dog can empty a frozen XL Kong in under 20 minutes.
I do the microwaved egg one too, but mix the egg with a bit of kibble as well, microwaving in 30 second bursts, squishing it down between each burst, before cramming in more kibble and wet food on the top. You do have to be careful to give it plenty of time to cool though.
The one that's taken him the longest to eat was a dried tripe stick pushed through the inside and out the hole at the other end. That lasted for days ... but stank to high heaven!
My older lad takes forever to eat them when wedged fish-skin fingers.
I once tried a fruit one, containing yoghurt, fruit and banana - my dog looked at me as if I'd gone completely bonkers. I do know lots of other dogs love them though.