I solved this problem by cutting the heated area into half by using a thin transparent foil you usually use for painting jobs.
If you reduce the room to be heated, you cut down the needed amount of heat. Learned this in the military with the poor insulated trucks.
Maybe this helps a bit.

 

Winter front is your answer. Mopar sells a really nice one that fits like a glove with adjustable velcro openings so you can leave it on with temp swings and have it opened up or closed enough for the right cooling at anything between -20F and +70F. I don't use the block heater until single digits and below. A few minutes of high idle before pulling out when it's really cold and I get heat in 5 minutes with the winter front closed:

 

My 98 wont warm up worth a damn in the winter if I dint block the radiator I have an Alaskan Tarp very nice to have. You can use a piece of cardboard to and plugin it in will help you out to.

 

They just don't make much heat unless you're putting the fuel to them workin' em'. You have to conserve what little heat there is being made running unloaded.

 

Ah, I've done the winter front, that didn't help either. Can't block off the area to be heated either, cause I haul the wife and 2 kids.....

I'm gonna look into a few options this spring to see if there is a better solution. Maybe time to do some custom fab....

 

How cold does it get. It got to Neg 29 here in colorado and my truck warmed up....

 

It will get down to -40 here. Usually sits around -30. And the ole megacab is a brick of ice.

 

It aint your truck its ole wheather causin your problems

 

Yeah, I know. This spring/summer is going to be concentrating on keeping heat in for the winter.....

 

If you want to spend money on something works for quick warmup, add an EB. Bonus is you have better braking also.