I abandoned horizontal pushups because i couldnt put any more weight on safely. I always did them declined btw, not with hands and feet on the floor. I was doing them with 20 or 25kg and i could go heavier but it was just dangerous and unstable to put on more weight. I had plates in a flimsy backpack and the whole thing was a hassle putting it on and off.
But now i just thought about something: i thought about using any kind of bag (rice bag, coffee, punching etc.) and make it weigh whatever you want, and then use ropes or whatever and hang it from something (parallel bars even, or just from the ceiling). The idea is to get under it and put it on your back and do pushups. This would be the safest thing i think because it is suspended and it cant fall off and you dont even have to take it off or put it on and spend energy doing that. You'll have to hang it so you can go the lowest you can in the pushup with the bag still hanging so the weight is never off of you.
This should work right? And i imagine you can use more weight than you ever could with other methods where you have to put it on somehow.