I've built a few 401 plinths, with no reason the designs couldn't be applied to a 301 (although I'm not conversant with the 301!).
The one I currently use is a two piece beech ply. The motor unit is located in the top piece, with the base section attached, but isolated, with rubber mounts. The arm board is designed to accept a 9" or 12" tone arm and mounted on a pillar that is secured to the base board. This results in no contact between the motor board and the arm board (except via the rubber mounts), hence theoretically reducing any vibration between the two.....!
Another recent one is also beech ply, but is a one piece plinth arrangement with a 'well' where the arm board sits. The arm board is supported by a fabricated pillar, also utilising rubber mounts to isolate the arm board from the plinth / motor and has no direct contact with the main plinth.
Both sound fairly impressive, but I've never had the opportunity to determine if there is any significant advantage in sound quality when compared to a conventional plinth.
Technics SL1000R, DynavectorXV1t, Garrard 401, Jelco SA750LB, Decca Reference, ATVM750SH, AT33Mono, Hana Umami Red, Reimyo CDP777, EarYoshino 912, Ear Yoshino 509s, JBLK2 SL5800s, ART Dram interconnects / speaker cables.