In the picture above you will see a warboss with the "head Whoppa" since there was no model to indicate that it was that weapon I orkified a space marine dread axe to be the head whoppa.
In this picture you see a big mek on a bike with Kustom Force Field.
This is a picture of my Ghork Rider. A warboss from a few years ago that didn't have a model, so I made one. I changed the name to Ghork Rider, but explained to my opponent the rules I was using for him and showed them the rules in the book. (pictured with deff copta for size comparison).
All of these use the "Counts As" rule. Just explain to your opponent what the model is before the game. Also it has to be "close enough". That means you can't use a grot holding an armor plate to count as deff dred, but you could use a toilet paper roll with arms and legs glued on as it is close enough.
Now, I haven't been in the tourney circuit for a few years now, I am not sure about "counts as" in the tournament arena, you will have to chat with the organizers to see if your model will be legal there. If you are not playing tournament then "counts as" models are usually just fine at your LGS and in the garage games.