Kill-Roy.It's not that hard to build a drop in you could definitely definitely do it! My friends and I made one using old wood we found around which was a little sketchy but it worked out fine, and we just used old carpets to get speed so it cost basically nothing.
You could throw something together very easily with some old wood and some creativity. My dad is a contractor so when I told him I was building this he made sure I did it "right."
Idk what kind of things you have laying around but an idea I've wondered about is using an old slide from a swing set and using some kind of turf/carpet on it. If it's wide enough that would be dope for a drop in haha. You'd need to figure out something for the lip though. Also if you have a deck you could build off that. I used my deck as a ladder. There's a bunch of things you could do.
Also, I would highly recommend having a flat(ish) lip. Mine was built too much like a lip you'd see in a park and it bucks you back pretty bad so I needed to rebuilt. So i guess what I'm trying to say is if your rail is 18inches tall(decent height for a flat rail) build your drop in to come up about 12 inches off the ground and have a flat lip that rises a couple inches. If you want it to be ride on then build to like 17-19 inches. if you want more of an urban lip, then leave if flat with no lip.