Whatever you do, don't put rocks in them :) They will just take up space and reduce the space available for materials that will break down and produce nutrients. I know it is hard when you have limited money with which to fill raised beds, and I always have the same concerns about the "wood" compost, but I am filling some new beds myself and have a sort of method that is working out for me. It is based on the newer method of raised beds called "hugelkultur." I have a couple new 4x8 beds that are 12" deep. The "soil" they sit upon is really "fill"--gravel and sandy sterile dirt put there when the house was built I guess. So it is hard and not useful for gardening, but will drain fairly quickly. I built a bed, then I dig down into the rocky "fill" inside the box another 12" below grade (this bed will be for veggies with large root systems eventually). Then I filled that space with pruning wood from an old apple tree I have, stomp it down and fill it again and stomp it down, so there is an almost 12" deep layer of wood.

building the bed and part of the pruned wood I will fill it with

I don't have photos for the other layers, but then I added a layer of horse manure and alder sawdust. The horse manure is from a neighbor (look on craigslist under "free" or "farm and garden") and is not completely "finished" composting and neither is the sawdust, but it doesn't have any manure or ammonia smell anymore. I added some compost from my own pile, but that isn't completely composted either. The top layer is only a few inches of whatever *good* soil I can scrape together--usually a mix of peat, old container mix, vermiculite, etc. Then I add amendments (= fertilizers) that I buy in bulk by the pound by the local eco-groovy farm and garden store: a nitrogen (N) source like fishmeal, bloodmeal, cottonseed meal, bat guano, or alfalfa dust; a phosphorus (P) source like bonemeal or soft rock phosphate; a potassium (K) source like greensand, and other rock dusts, kelp meal or whatever for micronutrients and minerals. The best "top layer" if you had nothing to work with would probably be a mix of purchased material , if you had the money, like a good compost mix, peat or coir peat, and something for drainage like perlite or rice hulls or vermiculite. Plus amendments for N, P, K, and micronutrients as mentioned above. A little goes a long way. Extravagances if you had the money might be worm castings, biochar, or some really nice bagged planting mix with compost. Filling a 12-inch deep 4x8 bed takes a full yard of soil, so it is too expensive for me to buy that amount of good growing mix at the moment. The wood acts as a water reservoir as it rots and gradually holds more and more water after the winter rainy system and gradually composts down there. It also probably has air down there that helps. The grass clippings and manure are both high in nitrogen that will help balance out the carbon in the wood to compost down. I honestly didn't think I would grow much in this bed in the same month I filled it, because there was only a few inches of soil before the more unfinished materials below, but I planted fast-growing green veg in the cabbage family that like high nitrogen in it: bok choy, broccoli raab, kale, and mustard greens. These are absolutely thriving--here it is 35 days later and this is after I have been eating a lot of the greens already:

Only the 4 plants in the far back corner were planted as starts. The rest were all directly seeded into the bed. I estimate I have eaten at least twice as many greens from this bed already as you see presently, by harvesting/thinning plants and cutting leaves from plants as the babies sown in between the rows start needing sun. I could actually have five productive rows in here and probably eat greens every day or two and they would be replacing that growth almost at the same rate. These plants in the cabbage family can be harvested only about 35 days after planting from seed--super productive and super healthy either raw or in stir-fries or added to pastas, etc. The more bitter ones, like broccoli raab and mustards (Brassica rapa) are very yummy when quickly sauteed with chopped garlic and some olive oil, a little squirt of lemon after, maybe some chopped nuts, and added to pasta or eaten alone like that.

Here is what the bed looks like 24 hours of the day apart from when I water it, using just black poly pipe from Lowes or Home Depot for hoops

If it's not covered, cabbage family plants here get decimated by the little green worms that come from the white cabbage butterflies. Aaaand, I'm going to do a second bed like this and plant with non-cabbage green veg: lettuce, chard, orach, spinach, cilantro, and basil. It won't have to be covered for the worms, but will be covered the same to protect from deer. This works as a mini-hoophouse and can be made warmer with perforated clear light plastic row cover, or for overwintering greens can be covered with greenhouse film purchased by the foot.

My tomatoes this year are going into large black plastic nursery pots, with a bottom layer of rocks for drainage and to fill up extra space that the tomato roots probably won't need. Maybe a layer of mesh over the rocks for easily removing later. Tomatoes take a huge amount of raised bed space and the soil between is pretty much wasted in terms of producing green edible food. That's why I usually do them in pots or grow bags. I'm trying to do whatever is most efficient use of space for producing the most food, and the cheapest right now.

Any of the 6" deep beds would be perfect for strawberries--delicious, superfood, produce all summer into fall, probably enough to feed you a meal every day. I am expanding my strawberries yet again this year. Lots of production, just like the brassica greens and non-brassica greens, and the tomatoes once they are producing.