SketchUp is free for personal use. Someone said the best things in life are free. Well, they didn’t mean software but it is close enough. Yes it takes time to learn how to use SketchUp but you don’t have to be an expert. Sizing 3D shapes of elements you want to use in your project and putting them together to create a mockup is almost enough. I highly recommend it. It was invaluable tool when designing and building my lower kitchen cabinets. I wanted to fit a fridge, water heater and water tank inside of my kitchen cabinet and still have space for couple of spoons. It took several different ideas before picking my final design. It was a fun project and my design looks very promising.
TIP. Watch some tutorials first. It is very tempting to just learn as you go, to try to figure it out on your own. At the end you save time and have more fun if you have some skills.