It will work best if you sow your seeds after the coldest weather has subsided which will give you an early start by a few months. Make sure though that you can keep it cool (ventilated well) though - doors that open on each end will be needed.

All in all, a small house will heat up fast and become too hot without proper ventilation, fans, etc even when it's cold outside but sunny.

You'll want to seal it in white propagation poly as well and I'd only use one layer. 3 mil for the sides will last two years and some wavy plastic sheeting for the roof tightened down with pole barn screws. You're not building the Tag Mahal here.

I'm no expert but just happened to chime in first.

Plans, "schmans" - if it's small, get out the hammer and nails and frame that baby up. Look at how a garage is framed if you need to.

Dax

The main problem with sensative crops like tomatoes is that you will need to constantly monitor temperatures in a smaller enclosure. One month gain in starting is a more realistic under most conditions and then you would be best to germinate tomato seeds indoors at much higher (80-85F) temperatures before placing seedlings in the frame.

A few thimgs that are useful for a smaller unit:
For an aid with heat a light bulb or string of Christmas lights may make a big difference on cooler nights. For daytime a hinged cover that can be propped at various heights is helpful. If you have an old waterbed or swimming pool bladder you can lay it inside and partially fill it to buffer the heat extremes.

A thermometer laid inside will be helpful during your frequent evening and early morning visits when you have the unit tightly closed.

Cold Frames tend to work best for hardier crops like cole crops or onions. But if that is what you have available you can easily make it work for almost any crop. Years ago they were the plant starting structure of choice for most home gardeners.