I am having more difficulty making a sled for the acrylic than was expected. Using the Tips & Tricks (Lithopane (Lithophane) Carving 101) method of carving out a recessed region for the acrylic to sit it has presented some challenges. I am using the latest version of Designer. To my way of thinking, a straight router bit would be the best but Designer will not allow me to choose the bit and wants to do the cut with a carving bit, which is a hour or more. Best was over 3 hours.
In Designer, I first made a rectangle of the blank size, then a carve region, setting the depth. What am I doing wrong?

I can route a recess with my router in about a half hour, including setting up edge guides.