I just got the cheapest 2x6 they had at Home Depot. I can't remember if it was pine or cedar, probably pine, but it doesn't look at all rotten after one season and being watered basically every day. The screws didn't hold the box together very well, but that may be because it was ran over by a tractor on one edge...

I'm going to put a wooden stake in each corner and put a few screws through the box into those and hopefully that'll hold it together better. I'll try to avoid letting tractors drive over my box this time around though.

As for the oils, I don't know. I didn't treat mine with anything but I didn't really care what it looked like or how long it lasted. I assume they would work, but I would wait until somebody who knows what they're talking about replies.

Box home supply stores all have a discards bin. In it they put wood that is short or damaged. If the garden is going to be in the back 40' instead of in the front yard than this wood is perfect for your needs without $$$$. One inch or two inch thick 4 foot boards are sometimes available. If not today than tomorrow. You might find 3 today and 3 next week, and 2 the week after. Check in whenever you are in the area. You should find each side for about .52 cents each. Preserving the wood is not so critical at that price. The hardware to connect may $$$. Maybe instead a 2X4, one in each corner,cut to the height of the bed and connect the boards to it. $3 might be all that is necessary to pay for a box. If you have access to cinder blocks or another edging, logs, that will work fine too. A 16 square foot garden does not have to cost $168 unless you want it to. If you can't find discards than the least expensive wood is fine. My transplants did not do too well in just leaves and grass clippings on top of beach sand. The leaves may be more composted next year and do better. I would add some charcoal or biochar to get the leaves to break down faster.

Another way is to square foot garden with out the raised part. The thought out spacing and not stepping on the soil are maybe more important than the raised part. Adding amendments with out digging in and planting in that above the poor base soil in a square foot non stepped on pattern is most of the way there.

I used pine 2x6 boards cut to 4' lengths. They weren't too expensive to make 3 boxes. Cedar is way expensive for a trial run.
My hubby connected the pieces with 3 deck screws, going in at an angle, two one way and the third the opposite. That's the best I can explain without a picture. They have held together very well this year. Since I didn't like the raw lumber, look we treated only the outside and top with deck stain.
Goo luck,
Keski

You may want to add something besides just dirt & leaves. Leaves are good but in composting, they are considered a "brown" (carbon). You'll probably want to add some form of "green" (nitrogen) to go with them. Grass clippings, coffee grounds (you can get them at Starbucks for free), etc.

You may already know this but I just wanted to mention it in case you didn't.

Here is a link that might be useful: Tales of a Transplanted Gardener