The path in my hoop house is a little wide, which was intentional both for access to the back of beds, and because I knew I'd use the space for spring propagation (And, I want to fit a lawn chair in there!). Still, there are times of the year when I wish for a little more growing space. In addition, I have a hard time germinating cool crops like spinach and lettuce in September, which is when you need to plant for overwintering--in our area the hoop house is pushing 100 degrees that time of year, even with the ends off and the sides rolled up. So, I had this idea to make movable grow-boxes. They can be set in a cooler spot for fall crops, and in the late winter, started indoors.
While you can probably make this out of pine or any scrap (untreated) wood, I turned to my favorite cheap building material: cedar dog-eared fence boards (see also my compost bins). For one six foot board you can make exactly one box with no waste, that happens to fit nicely in a standard size flat.
For the handles I used a doorknob drill bit. And I paired up the dog-ears to make matching ends. There are a few holes drilled for drainage as well (the first batch I drilled in the bottom, the second pair I tried the sides, we'll see which works the best).
My only minor complaint so far is that the handle height makes them a little bit tall for putting under lights, my spinach is stretched a bit. Still, the plan is to get them outside as soon as possible after germinating--these should go out later this week, once they have their first set of true leaves.
Alternatively, you could size these up to fit a window box liner, and you could also get fancy and make the ends tapered a bit. I've seen thinner wood used to make standard-size reusable flats for starting plants. I think they would make a great kids project (after the prototype, the second pair took me less than an hour start to finish). They could also be sweet for pre-planted salad boxes to sell at a spring farmer's market, or to start forced bulbs in.