SP-10 Mat

Have one made from Delrin.

I second either the Boston Audio Mat2 or the SAEC SS300 mat, with a slight nod in favor of the BA one. I use it on my SP10 Mk3.

No issues with the weight of 870 GM's. In the day the SAEC was used a lot on the SP10's.

Hi Lew, never tried my Boston 1 mat on my Exclusive P3, maybe I will :-)

I have had much experience with both the Final Audio copper mat 4.5kg and the Micro Seiki 180 copper mat on a variety of TT's. Both of them sound significantly better when used with either a heavy stabiliser/washer or reflex clamp/washer.
I have both the Final Audio stabiliser (1.8kg) and Goldmund Reflex Clamp - different sounding but both are good. The Sota and Simply Physics Reflex clamps would be worth a look. You get the benefits of the copper and clamping properly takes the "edge off".
I also think the use of the copper mat on the SP10 provides a bimetallic dampening effect on the platter overall. I prefer it in this application to an acrylic mat.

Downunder, I used a BA Mat1 for quite a while on my Lenco platter. It's excellent in that application, but I found that the Mat2 is far better than the Mat1 on my SP10 Mk3. I have not tried the Mat2 on the Lenco, because now the Mat2 "belongs" to the Mk3. I would guess you will like the Mat2 better than the Mat1, for your P3. (By the way, if you hear of a P3 for sale, let me know.)

One thing I don't like so much about the BA mats is that they tend to slip against the platter. That is not good for those of us who worry about torque and stylus drag. What good is a powerful motor if the platter mat is slipping, even a tiny bit? So I use a very small piece of Scotch Tape along the rim, to prevent slippage on the Mk3. On the Lenco, I used double sided tape, between mat and platter.

Dover, I never met a record clamp I could really love. I do occasionally use the SOTA clamp, but I almost never actually clamp it. I just sit it on top of the spindle so as to dampen vibrations just by virtue of its mass. Yes, it's rather light in weight for this application, but I always have found that the really heavy gravity-dependent record weights suck life out of the music. On the other hand, I don't own any of the very heavy copper or other metal mats. The SAEC SS300 (my only metal mat) is quite a bit lighter than the CU180. I think it weighs only a bit more than the stock rubber mat made for the SP10 Mk3. The Kenwood L07D uses a 5-lb stainless steel "platter sheet" that is part and parcel of the platter itself, altho it is removable. I don't like the OEM Kenwood record weight either, for its effect on the sound of the L07D.