I had a recipe I knew inside-out, I knew exactly how tall it would be in my usual pan at a certain temperature so I tried this method and it works perfectly for me for any size pan, even novelty pans.
Ive attached a simple spreadsheet but I shall endeavour to explain it here too:
Measure the height of the tin you know works perfectly
Fill it to the brim with water (or if its a lose bottom line it with a bag and fill with flour)
Record the weight or volume of water/flour it takes to fill the pan
Write this number down, for example it takes 1000ml to fill my usual 3 tall pan.
Now say I want to use the same recipe in a much bigger 3 tall pan; it takes 2800ml or water to fill to the brim, so to work out how much I need to scale up my ingredients I divide the new pan size by my usual pan size (2800 divided by 1000) = 2.8
So I need 2.8 times more of each ingredient
The attached spreadsheet will do this for you
I find it works better if you scale up this way in single layers rather than a full 3-4 inch deep cake you intend to slice as you need to reduce the oven temperature a lot more with full cake recipes.
Each time you measure the volume of a new pan write it down, that way you dont have to do it again and again!
For novelty pans you clearly cant make them in layers plus they arent a uniform depth so estimate a depth in between the very highest point and the very lowest and keep your estimate on the deeper side as you can always trim but you cant add to a baked cake!
if your pans are different height you may need to take account of this- the spreadsheet attached will do this for you but don't scale up to a 2 inch tall pan if your original recipe makes your cake 3inches tall- divide your original recipe in 3 and make three seperate 1inch layers in your new pan!
This also works for the amount of filling youll need to use (I do find it overestimates filling slightly though)
If I think of any more tips Ill add them to the post
Hope it helps someone!