Important characteristics; found slabs for bonsai

Hello all!
I'm enjoy fishing & this has brought me to some interesting areas. One of which is narrow but, long fast running river w/in the forest (Maine) there are various height waterfalls & some dry waterfalls & dry streams that once connected to the river.
One area has many pieces of thin slabs of rock (all I know of geology I leaned in the 7th grade! I know yearn
to learn more & how to distinguish rock types) I've seen trees & forest plantings on similar slabs. I've got a husband & a drill & many possible candidates.
What should I look for when choosing? Is there a certain kind of rock best for these?If the tree has an 8" canopy (random example) should I have 1" of extra space on each side? Of course, it's relative to style & personal opinion too. . . .
Is there a suggested thickness? This too will be affected by taste but, I assume it must be thick enough to be stable & strong yet thin enough to drill through.
Suggestions on size bits for drainage holes? I'll decide where the holes should be upon deciding placement of tree. My most mature tree has a home & this is simply for future use not a specific tree. Having a specific tree in mind would probably make this easier but, if I find a particularly nice & suitable slab ill keep it for the future.
This may be an abomination of display but, when I have company I put my willow leave ficus atop a piece of such slab. The bottom is perfectly flat so it sits nicely & the edges are interesting. Bonsai stands are quite expensive from what I've seen. Being a "newbie" all my $ has been used on material, tools & books. I'll have to get creative until I can afford such.
Any thoughts/ideas/comments appreciated!!!!

Maybe I should've put this in the pot forum??
Apologies