Personally i would get a company to CNC machine & polish a piece. There is nothing worse than hand finishing IMHO. Looks a million times better done with CNC. In fact you do nOt have to stick to Slate, You could use Marble or granite. Just make sure you get both faces polished as a lot of places only used one face polished stock.. I remember J A Michell used to supply the Elec Reference deck in Marble, Bootifull
I found an interesting little online mag/blog called MOOF - moofmag.com/ It's not strictly prog, in fact it describes itself as "psychedelic music & arts magazine with a penchant for the more obscure" but there is a bit of crossover when thery're discussing bands like Comus and Jan Dukes De Gray who sort of sit on the prog/folk border, although the actual article references acid-folk. Their "album of the week" in the July issue was released in 1970 - Sweet Smoke's Just A Poke - although there are a couple of others also laying claim to the album of the week title, by Julie Tippets (1975) and Kevin Ayres (1969).
Very Early prog takes its roots fro Psychedelic Rock/pop, A very fine line that a lot of Crossover prog bands could not fully shake off.. A hell of a lot of those bands called it a day after one album so never saw any vision to Progress fully. When i refer to Psych i dont mean Flowers in your Hair/ Acid etc..
This is an example of a band crossing over from Psychedelic pop style to early prog.
FLAC & Sony 'CDP-XB930E' UK Special Edition into Caiman 'SEG' DAC..AKG 'K92' Heasets