I am a novice woodworking hobbyist with Poland syndrome. For me, Poland syndrome means that my right arm is shorter than my left, my right hand is smaller, and my fingers lack the knuckle joints that most people have. Due to these limitations, I cannot operate a miter saw trigger grip with my right-hand. Nor do I feel safe trying to operate it by reaching across my body with my left.

A quick search for “left-handed miter saw” on Google led me to some discussions about the lack of left-handed tools and the need for lefties to adapt to a right-handed world. Unfortunately, adaptation isn’t an option for me. Does anyone know of any miter saws that might meet my needs and/or any tool manufacturers that build left-handed or accessible tools?

If not, can anyone advise me about how to better use the tools I do have?

Here are the details of my current project:
My roommate and I have created a 6-piece mosaic of mixed tile and stone. Each piece is on Hardie Board with almost no margin beyond the edge of the outermost tiles.

Our original plan for finishing this project was to use 3/4″ pine outside corner trim to build a low-profile pseudo-shadow box. We thought this approach would require minimal effort in that the trim is mostly finished and we could glue one side of the trim to the back of the Hardie board and fill in the gap between the edge of the Hardie board and the other side of the trim with grout to maintain a consistent look with the interior of the mosaic.

Unfortunately, this approach didn’t work because we have don’t have a clamping system that will allow us to get a tight miter while adhering the trim to the Hardie board. Next, we decided to try to build a test frame by itself and, if that worked, to build the remaining frames slightly larger than the mosaic pieces and then insert and adhere the Hardie board to the completed frame.

We have a miter box with a back saw (like this one: https://www.google.com/aclk?sa=L&ai=DChcSEwjmpLLEgqjpAhXCPa0GHbfLDTYYABAcGgJwdg&sig=AOD64_3H-w3NfrM7r4cSeAEiiJyAXyRuEw&q=&ved=0ahUKEwix6K3EgqjpAhVIsZ4KHXtUBu0Q1CkItAQ&adurl=) and the two of us tried cutting the trim pieces for the test frame together, but are struggling to get the 45-degree edge perpendicular to the flat of the sides and, therefore, are having trouble getting a clean miter.

Project-Related Questions
1. When I talked to a Home Depot employee about setting up my miter box and clamping the material I was cutting, he asked why I would clamp it and said I should just hold it with my hand. I did not discuss my physical limitations with him. My roommate and I have struggled to keep the trim stable during the cut (with me holding the material in place and my roommate sawing).
1a. Are we really not supposed to clamp material when cutting it in a miter box?
1b. Is there a technique to using a back saw that we could benefit from learning?
2. When we saw the 45-degree cuts weren’t always vertical, we considered and began sanding down the “high” part to achieve a better join, but found our sanding wasn’t flat.
2a. Is there a recommended technique for sanding to a specific angle and, if so, what’s the recommended “extra” amount of material to include in the cut to ensure the target length?
2b. Could my hand-plane be useful in this scenario even though the planing would be cross grain?
3. I know we probably made many mistakes in our initial approach to this project and we are very open to all feedback, but what we would most appreciate is advice on next steps for completing this project.

If you would like me to post any pictures I am happy to do so.

Thank you in advance for your time and feedback!