Allot of city and county culverts and river banks are built up with cement sacks. I did the same with mine. You just get the smaller sacks since the height of the sack is what's important. A 40 or 50 pound sack is just as tall as an 80 pound sack, but not as long. No need to pay for a wider 80 pound sack for stacking them.
Dig back on both ends of the culverts. It's just as important to protect the back end as it is for the opening. Then start laying them out. Once you have them in place, hammer lengths of rebar through them. This will tear open the sacks and allow the concrete to mix with one another and it will also lock it all together when the cement sets.
If I have water available, I'll dump a few buckets over the sacks to get things started, but in time, they will get enough water to cure on their own. Just don't do this right before a big storm.
Here's one of my culverts.
Eddie