I'd steer clear of clover. It can be tough to get rid of. It's great for pastures and places where you graze animals. But you want to be able to clear a raised bed and keep it clear.
This time of year, I tend to use a standard mix of about 10 to 20 different plants, many of then nitrogen fixing. I plant both cool season grasses and broad leaf plants (usually legumes) as well as some warm season grasses, grains and broad leaf plants. I'll use the same cover crop mix (innoculated with beneficial bacteria) on my raised beds as well as throughout the entire garden and orchard. I mix it in a 5-gal. bucket and then broadcast it into the mulch before lightly raking it in. Usually, I'll buy my seed from
https://www.greencoverseed.com/
No -- I don't work for them and have no affiliation with them. They've taken too much of my money over the years. I should own stock in them.
For cool season, I like Common Vetch, Hairy Vetch, Chickling Vetch or Purple Vetch, Spring Oats (or any other oats), Cow Peas, Pearl Millet or Proso Millet, Bell Beans, Chick Peas, Mung Beans, common alfalfa, Sunn Hemp, Buckwheat,
Flax, Spring Lentils (or any other lentil—even a bag of them from the grocery store), Cereal Rye, Sorghum, Collards, and various radish seeds (whatever I have around). Any mix of peas or beans that will germinate go into the mix. Sometimes I'll sprout the legumes in a jar of warm water on the kitchen counter, and then gently till them into the soil with the rest of the cover crop mix, just to assure germination.
Terminate your cover crop before it goes to seed, or you'll lose a lot of that precious nitrogen that the plants have fixed. This might be tough on a raised bed, but if you could find a way to tractor chickens over the top of the bed, you could let the girls do the tilling for you. Maybe create some sort of mobile cage out of PVC and some inexpensive netting. All that cover crop biomass either goes into the
compost pile or is directly laid down as a mulch in places where I need some soil cover. Nothing goes to waste.
If I know that I'm going to be transplanting whole plants into a bed (like a tomato or pepper) and not planting seeds, I'll take my electric hedge trimmer and cut down the cover drop that way. Zip -- it's done. With a 4 foot wide raised bed, I just walk down one side, and then back down the other -- like a big green haircut. That way all the roots stay in the soil where they belong and the nitrogen doesn't go away. If you are planting seeds, it's better to yank that stuff out, as it's tough to plant evenly with all that root mass still in the soil. After you've composted the cover crop, you can return that compost to the bed as a top dressing.
Best of luck.