Hardening off plants before planting them in the garden is important. Instead of carrying all the seedlings in and out for a couple of days, we like to use an old window temporary little greenhouse and frost blankets.
The idea was to use what we have and make it as simple as possible. It worked. In fact, I really like it. We had some old light-weight windows and some cinder blocks in different sizes. We used the wider ones for holding the window frames in place on the outside and the more narrow ones inside. The greenhouse is placed right in our sunny but sheltered driveway.
The top of the little greenhouse we covered with a Frost Blanket. It gives some shade and protection from wind and cold, but the fabric lets air in, so we do not have to worry about overheating. The blanket is held in place simply with some clothespins, those are handy in house and garden :).
Since some nights are still very cool and rainy we added another layer of protection of bubble foil/wrap for the night and if needed. Again holding it in place simply with some clothespins.
The tomato and pepper plants love it in there, and we are happy to have them out of the house. A simple solution that works very well for hardening off plants and even keep them in there for longer as needed. The natural light is much better than anything indoors. Before planting the plants into the open garden we take the cover off for a couple of days so the plants get used to the direct sun and wind.
The temporary little greenhouse worked so well that we upgraded it to a raised window protected garden that we use in spring for hardening off seedlings and in the summer to grow heat-loving plants in.