In order for it to look alright you're going to need to do some work with fibreglass. This is due to the contour of the cover, since it has those recesses keep loose items from rolling around. Round speaskers probably fit these quite nicely.
I made my own cover out of a sheet of MDF and some dowel to angle the speakers.
This is how mine is setup.
oh man i dont know if i want to get into something like that. i have no clue how to do fiber glass work maybe ill just do some
6 1/2s back there and maybe a component system.
It wasnt difficult at all really. This was my first attempt at using resin and I feel it turned out pretty good. I just didnt use the right fabrics for the base and covering.
But if you really dont want to do that, then circular speakers are the way to go.