I like it
Also , you might find them stretching in a week , and you are going to freak out , so if i can borrow foutwenty idea anf make a hybrid f1 idea

( first idea)
You can do what foutwenty said , put the seedlings halfway in the cup , in the plug , and when it starts to stretch to like 4 inches and you have to manage it , you can then , just dump promix etc in the cup , filling it up and making a nice comfy soil fill up for your stretchy seedling , in effect burying 2 or 3 inchrs so the seedling is deeper in medium again and now is back to two inches tall
Of course thats only necessary if things go haywire , like heat issues that cause the plant to stretch or light issues thst cause the seedling to stretch , hopefully you wont have those stretch issues , but normally there are beginner seedling issues.....
You will see , but they look good for now
If you put in solo cup make sure you punch holes in the bottom , my favorite method is take a crappy fork , metal , that you can heat up on the stove flame and tap the bottom of the solo plastic cup ,very effective and fun
You heard it here first !!!