(just to give credit where credit is due, this wasn't my idea or anything, it came up in a thread not too long ago and this was the solution that i thought seemed best/easiest.)

Guess I should have included more details. My kitchen company has gone bankrupt. Thankfully, we had the 4.5" toekicks in our possession before they went under. So ordering taller toekicks or skins is not an option (that would have been my preference- I was hoping that the ones we had were 5", but no such luck). I do have at least one if not two extra lengths, so could cut slivers like madeyana did. I do have a can of matching stain, and actually a sharpie that's pretty darn close in colour. DH is an engineer, so he's fairly competent with scribing and figuring out how to cut odd angles, etc. Would love to hear from more of you who have come up with a DIY solution. Pics would be awesome if possible! Live Wire brought up a good point though. I was thinking I may ask in another post. But here I go... What do people do for the end cuts? I have three that will show. They are all at right angles to a cabinet that goes down to the floor. The peninsula end is just a straight 'skin', no rails, etc. WE do have extra filler strips that are the width of our stiles. I was kinda debating whether or not to have DH rip some in half to see if we could construct a 'fake door' panel on the end. But we don't have those ends on any other cabinet end runs... time to go to the FKB and see what others have done to finish off the end of their peninsulas. Could get our friggin carpenter to make something...I suppose... but he's taken three months and still hasn't delivered cabinets he said would take a few weeks. Just not fun anymore...I'm so tired... :>(

Thank you everyone. I really do appreciate--and take to heart--the advice to just replace the door. It's clearly wise counsel and I'm currently looking for doors. But I also think the heart of the original question is being lost in the door-replacing zeal. Even if I do replace the door, I still need to find a way to heat that part of the kitchen. That door is on the kitchen's exposed wall, so whatever form of heat I ultimately place in the kitchen ought to be near that door and that wall. Right now I am just trying to find the most energy-efficient form of electric heat for the kitchen in that spot. So, all things being equal (and assuming I get a nice new heat-tight door), what's the most thermodynamically efficient approach for that spot--toe kick, floor insert convection fan, or caddy-corner baseboard? Thank you very much.

@Seabornman I will have AC as I'm having a heat pump installed, but I think I'd probably have the vent closed when the AC was on, because I hate having it on my feet. Would you still recommend using an insulated one knowing that? Also, the rest of the ductwork isn't insulated. Is it something I should look at doing before summer hits? @mike_home those toe ductor kits are the first thing I found when starting to research this, but as you noted, it did look like something to use before the cabinet is installed. It was watching the install videos that led me to my original question on this post - do I even need to add a boot on either side of the ceiling/floor, or could I just run one piece of flexible ductwork that I could flatten a bit at the end to fit. Like how the toe ductor vent is more oval shaped at the end. @Mark Bitman why is flexible ducting a problem?