I think the Dynamite will work for you. Pick up a 3 gallon at a nursery and choose between one and three branches and remove the rest and any subsequent branches that will come up. It will grow as a tree form that way.

Here is a lik with picks of the single trunk...

Here is a link that might be useful: single trunk crapes

Dynamite is a good one. The more recent Arapaho and Red Rocket cultivars are also worth a look. The linked site mentions another I don't recall seeing before: Centennial Spirit out of OSU.

Here is a link that might be useful: Red-flowered Crape Myrtles

There are quite a few posts about this topic (dark red crApe myrtle cultivars) in both this forum and others. I'd recommend you do a search for something like 'red crape' and include all forums. You'll get quite a bit of useful info including people's personal experiences with different cultivars. In additions to color, height, and characteristic form, you might also consider info about disease (specifically mildew) resistance.