I really have Librarychick to thank for all the tips. I did it in less than 2 weeks. I didn't have butter or peanut butter, but I used a new treat. I reserved greenies for grooming sessions.There's a few things you can do to help that. My answer is always the same: cheat
Practice closing the door. Toss kibble in, and as soon as he's in close the door, just holding it closed without latching. He'll Hoover the treats down, and as soon as he turns around - before he can start getting upset - open the door.
Do that a bunch of times, until he's clearly less stressed when he sees the door closed. Like...5 times a day for a week. During different times of the day.
Get something super yummy like tuna or something else a little mushy. I'd use cheez whiz, or peanut butter just a teeny bit! Wipe it on the floor of the kennel, and repeat the process from above. It'll take him longer to eat the goodies, so he'll be in there longer without fussing.
Do THAT for a while. Part if what you're doing is removing the negative memories of being in the kennel and stressed for a long time, so do it lots over a span of time.
After another week you would start lengthening the time the door is shut with him aware of it. If he's still pretty upset you can either push more goodies through the bars, or go back to the previous step for a while.
When he's not as upset about seeing the door closed that's when you start adding time. Get him in the kennel with treats, as usual, then when he turns around wait 1-2 seconds and open the door. Do that 5-10 times a day for at least a week, then increase the length of time each week.
Once you get to 10 seconds with no fussing (if he gets upset go back a step for another week) you need to mix it up a bit; how long you hold the door shut should be random. One time hold the door for 1 second, the next time maybe its 7 seconds, then 3, then 10, then 2, then 8, ect. You never want him to think he knows how long the door will be shut, or he'll get upset as the time gets longer because who wants to be in a tony box for a minute or more. If he's never sure when the door will open he'll be more accepting.
Always praise him when he comes out of the box, but don't make it a huge deal or he'll fuss to get out for playtime.
The 1st time I had the door locked for just 1secs, he wasn't happy and refused to get into the carrier the next time. Instead, he went on top and behind the carrier looking for ways to get the treats which I threw in. Anyway, I still managed to get him to go back in. After a couple tries, lengthening the time the door was locked, he finally made it for nearly 2mins staying locked inside w/o a single fuss. But... I used upto 7 treats in all. I break one treat into 3, its a soft chew, and fed him through the hole at the top of the carrier. He seemed to enjoy getting his treats that way. He didn't wanna come out even after I unlocked the door but still closed, still waiting for his treats, but I really didn't wanna give him too much treats and kept repeating "no more", he finally pushed the door to get out.