Retaining walls - approx 18 inches - temp solutions?

I'm redoing a backyard on a row house. The yard is 19 feet wide. I had to dig down the yard about 18 inches in order to move the soil line down below the sill on the house. I'm going to end up having raised beds on the sides where I did not dig down.
For an event coming up in June I need to get some sort of retaining system in. I need something temporary because I don't have time to build the final solid retaining wall before hand. Any suggestions?
Thanks in advance.
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Rent some concrete formwork?
Don't forget that a ditch like that will be a hazard to partygoers.
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