Thorens TD124 with Fidelity Research FR-24 Arm help

You either countersink the base, or space the cart.
The weight, is just miniscule, if you trim your anchors,
lighten up on the mounting, hardware. The spacers are
just for that.  Soft to hard/metal spacers.
Poly, silicone, brass, copper.  They sound different too
You can tinker.

LOL I've made a few, crazy mounts for 121 and 124 with
16" arms.

Regards

You would likely get faster responses in the analog forum. I almost never come to this forum but was bored. 
I have a TD124 and mounted a few arms before coming to my last arm (I think), a Reed 3P. 
The TD124 has a very low profile due to the design of the chassis. Before I had mine overhauled and hot-rodded by Greg Metz of STS, I had to have a woodworker help me craft some armboards with a countersunk mounting circle drilled out of it. 
If you go to my profile you can see my Thorens now, with it's aftermarket tall massive platter which then needed the opposite-a machined block of alloy to raise my Reed. This involved an aftermarket bearing to support the heavy platter and I by necessity lost the platter clutch. 
The TD124 was designed for the tonearms of the day, which most often meant one of the SME 3009 variants. 
I hope this helps.