Spelling test trick
I've tried some different things in the past, like displaying the "bed" poster or coming up with other visual aids, but I found that most didn't really work for my 2nd graders. My suggestion is not a lesson so much as a simple reference I have them place on top of their spelling tests. I just have any kids who mix these letters up write "abcd" on the top of their spelling test, though you could do this for classwork or writing assignments too. Every student I've reminded about this will get in the habit of writing "abcd" somewhere on the top of their page and whenever the student needs a reminder they refer back to the top of their page. It's so easy and it always works - no fancy songs, mnemonics, or visuals needed - just the basic knowledge of their alphabet!
I've found it to be very helpful in assisting those kids who struggle, most of whom will grow out of this on their own unless there is an underlying concern. I think it was a parent who suggested this to one of my students a few years ago. It worked for her and for every other kid I've used it with so far, so I'm glad I stole the idea!