This is a rear speaker mount designed for the 4" Tang Band W4-1337SA.
It accepts square nuts (10x10x3mm - 10-32 Menards part # 85611). These are push fit and may slide along the radius line to accept variations in different 4" speakers and to compensate for different material shrink rates.
The design of the mounting plate is such that this may be used a backer in an 11" Ikea Blanda Matt Bamboo bowl to spread the pressure on both the hole lip and the OD lip of the mounting plate ($17 Ikea #602.143.43)
The terminal cup is designed to mimic the 4" Tang Band W4-1337SA so that the same flush cut baffle may be used
Use a SHARP chisel to remove the support from the 10x3mm slot.
Push a sacrificial nut through the hole from the outside. You may need to support the edge of the nut bracket on a table edge and gently hammer it in. Use a slotted/standard screwdriver to tap the nut out. Do this a couple times.
Mount the speaker to the mounting plate. If necessary use a stepper bit to open up the bolt hole bottom. Use 1 drop of CA glue at each outer nut corner to hold the nut in place. After 30 seconds add 1 drop of CA Accelerator. After 1 minute remove the speaker.
Acetone Vapour smooth for 45 minutes
Trimble (Nee Google) Sketchup Make 2015 with the STL exporter