I have never built or used a UDS but by the looks of your build you might want to add an adjustable air intake somewhere on the bottom of the drum that you can open while it is getting up to temp. I think why you had to leave the door open for so long is that the vents on the door were not letting enough air in. Maybe someone who has a UDS might chime in and disagree which will by fine by me but that is the way I see it by looking at the link with your build.
chargiller pro deluxe, weber gasser. Masterbuilt electric smoker