Good to know it works with analog pots. You mind find that the MCP45HV51-103E will work but only if you can deal with a 14 pin TSSOP surface mount part. Solder it up on a dip adapter and you should be good to go. The downside is you might have to hack a library to make it work, I don't know if the software requirements are different than what you're using now. I can't imagine it's that difficult, it's a control word and a pot value.
The other option (not as simple) is a rail-to-rail op amp that can use the +/-10 ref of the drive as a supply. PWM out of the Arduino into an RC filter and a X2 gain op amp. You could use any old op amp like the LM358 but you need a power supply higher than the output due to output stage losses. This is where the rail-to-rail devices help out but most are 5 volt supply, not as many +/-10V supply capable parts, almost all will be surface mount. The ADA4177 is one of many that should work but it's way over-spec'ed just to get to the supply voltage you need.