The dowel standoffs I used are just 1" hardwood dowels cut to the correct length, and drilled through with a clearance hole for #10 bolts that hold the engine to the firewall.

I added fender washers between the OS engine mount and the dowels to prevent crushing. The joint between the dowels and the firewall are OK.

BTW, the "correct length" depends on your particular plane. You have to measure the distance from the firewall to the front of the cowl, add 1/8", subtract the length of your engine and figure in the thickness of the fender washers.

The result will be the length of the dowels, more or less.

Here's some photos