What about the elliptic bases that come with the Khorgorath and Lord-Celestant in
AoS starter? They seem to be just perfect for both a Bloodcrusher (the one that's leaning the most forward also with the most outstretched champion riding it) and a Demigryph Knight, having enough space for it's outstretched tail to still stay in the light of the base.
I can't find my mournfang model, but if you glue it far enough back (and they don't have pointy tails, so it's possible) you probably will fit it on snuggly like the Lord-Celestant model.
That base is ten times better than anything round (mostly due to absolute lack of dead space on the model's flanks) and it will look awesome when you put a few of them in tight formation.
P.s. - I'm currently rummaging through my stuff, so if I find my mournfang I'll post a pic of it on that base.
Edit: Found the model, here's a comparison pic. Sorry for terribad quality of the shot, only had a phone camera at my disposal. Seems that it really is a go-to monstrous cavalry base. Definetely works better than any round one due to very little dead, unused space.

As you can see they fit very nicely on the base and only the pointiest tusks get out of it, the upward pointing ones stay fully within the light of the base. Also haven't noticed how goddamn huge the Lord-Celestant is as a whole - with his Dracoth he's almost as big as a mournfang cavalry model and they really do tower over everything else.