You're very courageous if you are going to make the plinth from raw wood planks. I created an engineering drawing of what I wanted and gave it to a "real" carpenter, who made the base for me out of solid cherry. (Sounds like you yourself have some real carpentry skills; I do not.)
I then mated that base to a slate plinth that I had previously commissioned. I bought the slate slab in PA, had it cut and honed by the company that sold the slate to me. They then shipped the slab to a water-jet company, also in PA, where the slate was cut to accept the chassis of my SP10 Mk3. The slate slab and the wood base are held together by 6 or 7 large bolts that engage threaded inserts I installed into the bottom side of the slate. Then the Mk3 chassis is bolted through both layers, top to bottom. The slate alone was "very good", but I think the slate + cherry is even more neutral. (What else would one say after so much time, effort, and aggravation?)
I then mated that base to a slate plinth that I had previously commissioned. I bought the slate slab in PA, had it cut and honed by the company that sold the slate to me. They then shipped the slab to a water-jet company, also in PA, where the slate was cut to accept the chassis of my SP10 Mk3. The slate slab and the wood base are held together by 6 or 7 large bolts that engage threaded inserts I installed into the bottom side of the slate. Then the Mk3 chassis is bolted through both layers, top to bottom. The slate alone was "very good", but I think the slate + cherry is even more neutral. (What else would one say after so much time, effort, and aggravation?)